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Is it possible to mark the protective MBR of GPT Disk as bootable from Windows ?

bootable protective mbr gptmbr.bin syslinux gpt hybrid grub2

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#1 devdevadev

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Posted A week ago

First I have used the interactive gdisk32.exe tool instead of sgdisk to mark the partition bootable.

gdisk32.exe \\.\physicaldrive3
x
a
1
2
w
GPT fdisk (gdisk) version 1.0.4

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): x

Expert command (? for help): a
Using 1
Known attributes are:
0: system partition
1: hide from EFI
2: legacy BIOS bootable
60: read-only
62: hidden
63: do not automount

Attribute value is 0000000000000000. Set fields are:
  No fields set

Toggle which attribute field (0-63, 64 or <Enter> to exit): 2
Have enabled the 'legacy BIOS bootable' attribute.
Attribute value is 0000000000000004. Set fields are:
2 (legacy BIOS bootable)

Toggle which attribute field (0-63, 64 or <Enter> to exit):

Expert command (? for help): w

Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING
PARTITIONS!!

Do you want to proceed? (Y/N): y
OK; writing new GUID partition table (GPT) to \\.\physicaldrive3.
Disk synchronization succeeded! The computer should now use the new
partition table.
The operation has completed successfully.
Press any key to continue . . .

then installed syslinux using 

syslinux.exe --mbr --active --directory /boot/syslinux/ --install G:

Syslinux installed successfully in GPT Disk but Why this command does not give any successful message in cmd ?

 

 

Write gptmbr.bin to MBR of GPTdisk as follows.

dsfo \\.\Physicaldrive3 0 512 GPTMBR.mbr
dsfi GPTMBR.mbr 0 440 gptmbr.bin
dsfi \\.\Physicaldrive3 0 512 GPTMBR.mbr
C:\Users\DEv\Desktop\AIO>echo.Installing Syslinux...
Installing Syslinux...

C:\Users\DEv\Desktop\AIO>syslinux.exe --mbr --active --directory /boot/syslinux/ --install G:

C:\Users\DEv\Desktop\AIO>ECHO Writing the special MBR ...
Writing the special MBR ...

C:\Users\DEv\Desktop\AIO>IF EXIST GPTMBR.mbr DEL GPTMBR.mbr

C:\Users\DEv\Desktop\AIO>dsfo \\.\Physicaldrive3 0 512 GPTMBR.mbr
OK, 512 bytes, 0.000s, MD5 = 2de3c0115d9de259bd986de75a184042

C:\Users\DEv\Desktop\AIO>dsfi GPTMBR.mbr 0 440 gptmbr.bin
OK, written 440 bytes at offset 0

C:\Users\DEv\Desktop\AIO>dsfi \\.\Physicaldrive3 0 512 GPTMBR.mbr
OK, written 512 bytes at offset 0

G:\boot\syslinux\syslinux.cfg

default grub4dos
 
LABEL grub4dos
MENU LABEL Grub4dos
BOOT /AIO/Tools/grub4dos/grldr

http://www.lightofda...cgi/BIOSBootGPT

https://mjg59.dreamwidth.org/8035.html

1. Start fdisk on the disk (fdisk /dev/sda) and ignore the warnings.
2. Type "a" to toggle the bootable flag, and choose "1" for the partition to enable. We choose
partition "1", not "3", because now we are talking about the protective MBR (that fdisk sees);
and there is only one partition there - the protective partition.
3. Save your work by typing "w".
$ fdisk /dev/sda
Command (m for help): a
Partition number (1-X): 1
Command (m for help): w

 

I know we can use fdisk under Linux to set the protective MBR partition as active. 

 

Can we not use DOS FDISK in order to to mark the protective MBR of GPT Disk as bootable from Windows ?

Any windows tool capable of doing the same what old fdisk of linux do ?

 

Regards...



#2 Wonko the Sane

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Posted A week ago

Your mission, should you accept it, is to write 0x80 at offset 446 decimal or 0x1BE hex of the MBR.

 

You can do it from grub4dos:

http://reboot.pro/to...ite-hex-values/

or from windows with *any* hex/disk editor.

 

The nice hexalter (site is dead, but available via Wayback Machine):

https://web.archive..../hexalter.shtml

https://web.archive....t/hexalter3.zip

would do nicely from command line to modify the temporary file "GPTMBR.mbr"

 

Only for the record, Joakim did make a nice little tool, but it seemingly went down the sinkhole :(:

http://reboot.pro/to...physicaldriven/

 

I might check if I have a copy saved somewhere, but do not count too much on it.

 

ONly for the record, these:

Expert command (? for help): a
Using 1
Known attributes are:
0: system partition
1: hide from EFI
2: legacy BIOS bootable
60: read-only
62: hidden
63: do not automount

are GPT/UEFI values they do NOTHING for BIOS/MBR booting.

 

:duff:

Wonko



#3 devdevadev

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Posted A week ago

ONly for the record, these:

Expert command (? for help): a
Using 1
Known attributes are:
0: system partition
1: hide from EFI
2: legacy BIOS bootable
60: read-only
62: hidden
63: do not automount

are GPT/UEFI values they do NOTHING for BIOS/MBR booting.

Before marking partition bootable I was getting 'Missiong OS' error if MBR boot from Syslinux (gptmbr.bin) based GPT Disk ? So how 2: legacy BIOS bootable flag do NOTHING for BIOS/MBR booting ?

 

https://www.syslinux...28gptmbr.bin.29

Missing OS (gptmbr.bin)-

 

The MS-DOS mbr (mbr.bin) of Syslinux provides the "Missing Operating System" error message while the GPT mbr (gptmbr.bin) provides the"Missing OS" error message. In order to fix this problem, enable the "boot" ("active") flag on the boot partition (in many cases /dev/sda1). At time of writing, parted-2.3 does not seem to do this job like expected. Instead, use "sgdisk" to get this task done.

If your first partition is the one that you are expecting to boot with the Syslinux boot-loader installed, then under Linux, one possibility to set the "boot" flag could be:

sgdisk /dev/sda --attributes=1:set:2

To confirm everything went fine run:

sgdisk /dev/sda --attributes=1:show

This should output:

1:2:1 (legacy BIOS bootable)


#4 Wonko the Sane

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Posted A week ago

Well, you posted about:

http://www.lightofda...cgi/BIOSBootGPT
https://mjg59.dreamwidth.org/8035.html

1. Start fdisk on the disk (fdisk /dev/sda) and ignore the warnings.
2. Type "a" to toggle the bootable flag, and choose "1" for the partition to enable. We choose
partition "1", not "3", because now we are talking about the protective MBR (that fdisk sees);
and there is only one partition there - the protective partition.
3. Save your work by typing "w".
$ fdisk /dev/sda
Command (m for help): a
Partition number (1-X): 1
Command (m for help): w
 

I know we can use fdisk under Linux to set the protective MBR partition as active. 
 
Can we not use DOS FDISK in order to to mark the protective MBR of GPT Disk as bootable from Windows ?
Any windows tool capable of doing the same what old fdisk of linux do ?
 
Regards...

THOSE commands in Linux will set the (ONE AND ONLY) protective entry in the MBR to "active" or bootable, which equates to writing 0x80 in the given offset.

I can only (hopefully) answer the questions you ask, not (yet) the ones that you fail to ask.

If Syslinux uses the GPT flag, it is ANOTHER thing, as the field containing it isn't even in the MBR partition table, but rather in the GPT partition table, on sector 2 and later ones

If you prefer, one thing is making active the protective MBR partition table entry (that is what the Linux fdisk command you posted does), another one is setting the "bios bootable flag" of one of the GPT partitions(which is what the gdisk32 you posted does).
 
:duff:
Wonko

#5 devdevadev

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Posted 6 days ago

Wonko the Sane, on 05 Dec 2018 - 3:29 PM, said:
Your mission, should you accept it, is to write 0x80 at offset 446 decimal or 0x1BE hex of the MBR.

The nice hexalter (site is dead, but available via Wayback Machine):
https://web.archive..../hexalter.shtml
https://web.archive....t/hexalter3.zip
would do nicely from command line to modify the temporary file "GPTMBR.mbr"

I don't know much about internal structure of Protective MBR so please teach me more about basics of Protective MBR structure in GPT disk, Terminology which you have used in mission and location of memory where value of bootable flag is set ? so that I can grasp my mission in term of real modification which linux fdisk do technically in protective MBR of GPT disk....Also provide exact 'hexalter' code which will write 0x80 at offset 446 decimal or 0x1BE hex of the temporary file "GPTMBR.mbr"..? Will same hexalter code be make Protective MBR of both 32 GB GPT Disk and 4TB GPT USB-HDD
bootable / Active ?

Wonko the Sane, on 05 Dec 2018 - 3:29 PM, said:
Only for the record, Joakim did make a nice little tool, but it seemingly went down the sinkhole  :(:
http://reboot.pro/to...physicaldriven/

I might check if I have a copy saved somewhere, but do not count too much on it.

Please find it ASAP for us...I need it badly...

cmdhexedit.exe mode \\.\PhysicalDrive3 0x1BE 1 0x80

mode = ?
At least explain about technical terms used in above syntax in term of Protecttive MBR of GPT disk...
https://thestarman.p.../PartTables.htm
http://www.diydatare...mbr_article.htm

Regards..



#6 Wonko the Sane

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Posted 6 days ago

I don't know much about internal structure of protective MBR ...


Actually you don't know much of the whole stuff.

BIOS uses MBR.

The MBR contains a partition table.

A partition table has 4 entries.

These can be 1 to 4 Primary partitions or 0 to 3 Primary Partitions and 1 Extended Partition.

Each entry is 16 bytes in length, they contain various fields, Active status, Partition ID, CHS start and end address, LBA start LBA length.
Active status is 1st byte of each entry and can have values 0x00 (non active) or 0x80 (active or bootable, that can be set ONLY for Primary Partitions) so it is at offset 0x1BE for first entry. (and respectively at 0x1CE, 0x1DE, 0x1EE for 2nd, 3rd and 4th entry).

UEFI/GPT uses NOT the MBR, but since it is possible that a GPT disk is connected to a BIOS machine uses the MBR partition table to "protect" the GPT partitioning.
GPT writes in first entry of the partition table of the MBR an entry called "protective" with these characteristics:
0x00 (non active) <- this is at offset 0x1BE
0xEE Partition type
CHS and LBA addresses spanning from sector 2 to the very end of the disk

 

The idea is that any BIOS (and any OS that relies on BIOS) will not do anything with a non-active partition in the booting phase and since any OS that doesn't understand GPT doesn't also know anything of a partition type 0xEE all the tools connected with disk use will not access that partition (and since the partition covers the whole disk, the GPT partitioning is "protected".

 

It makes normally no sense whatsoever to make the protective MBR entry "active" (via Linux or any other tool) because the addresses in that entry do not actually represent a volume or filesystem, they simply represent the whole rest of the hard disk, but that is what you asked and what in some cases it is actually needed because the BIOS (some of them particularly picky and/or badly written) "wants" to see at least one "active" partition when booting to process the booting code, as extensively explained on the very page you provided:

http://www.lightofda...cgi/BIOSBootGPT

Re-read it, with particular attention to  Note 1, Note 2 and Additional Note 1.

 

 

The set of commands you ran in gdisk32.exe do a completely different thing, they make a change in a field that is part of one of the GPT partition entries, the "attribute flags" field.

A GPT partition entry is 128 bytes long, the first GPT partition entry is on sector LBA 2.

 

This info is hidden (like the first google result for "gpt partition entry") on Wikipedia here:

https://en.wikipedia...Partition_Table

https://en.wikipedia...able_Scheme.svg

https://en.wikipedia...ries_(LBA_2-33)

 

If bit 2 of the "attribute flags" is set, then that GPT partition is "BIOS bootable" according to this:

https://en.wikipedia...ble#cite_note-9

http://t13.org/docum..._code_annex.pdf

 

which represents a "proposal" that NOONE (meaning the actual people that write BIOSes and Operating Systems) actually  EVER accepted/followed AFAIK, see also again the notes on the site you provided:

http://www.lightofda...cgi/BIOSBootGPT

 

Even if it is not an accepted standard the "proposal" (for the record by Rob Elliott from HP) makes a lot of sense and most probably the good Peter H. Anvin (the Author of Syslinux) seeing that it defined a flag for "Bootable BIOS partition" used it in the implementation of his gptmbr.bin.

 

:duff:

Wonko


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#7 cdob

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Posted 6 days ago

@devdevadev
Can you expain the feature request? What do you like to do?
Do you like to boot a USB disk at BIOS and at UEFI machines?

I understand: the 4 GB hard disk reports 512 byte per sector at the USB port
http://reboot.pro/to...-15#entry208337

What about a plain MBR partition sheme?
A 2 TiB partition and a second parttion?

https://msfn.org/boa...comment=1146124

Entry #1 -> "Start Address" = 2048 and "Size"= (2^32-1-2048-7)=4,294,965,240 (i.e. a number divisible by 8), these will be in hex respectively 0x00000800 and 0xFFFFF7F8 and 0x00000800+0xFFFFF7F8=0xFFFFFFF8=4,294,967,288 (<4,294,967,295)

Entry #1 marked as active.
Entry #2 -> "Start Address" = 4,294,967,288 "and "Size"="HDD whole sectors count" - 4,294,967,288

HxD.exe: lists "HDD whole sectors count"
https://mh-nexus.de/en/hxd/

BeeblebroxNT.exe
07 80 0 32 33 1023 254 2048 4294965240
07 00 0 0 0 0 0 4294967288 [Entry #2 size]

https://web.archive....beeblebrox.html
https://web.archive....bbdownload.html
The Windows NT/2000 version runs at Vista to Windows 10, however select admin permission yourself.


Some BIOS acces the first 128 GiB only: use a smaller disk
Other BIOS acces the firs 2 TiB.
UEFI machines should access the whole 4 TB.
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#8 devdevadev

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Posted 6 days ago

@devdevadev
Can you expain the feature request? What do you like to do?
Do you like to boot a USB disk at BIOS and at UEFI machines?

Yes, I want to boot 4TB USB-HDD both in BIOS and UEFI machines.
Actually I want to keep following partitions in 4TB USB-HDD.
 

Number    Start (sector)       End (sector)               Size         Code                       Name                                                                 
     #1                           63                     2047       1007.0 KiB       EF02/EF01      BIOS boot partition (GRUB2) / GRLDR.img (grldr)

     #2                       2048                 206847          100 MiB        EF00               EFI System Partition
     #3                   206848             67106815             29 GiB        0700               FAT32 (AIO, Multiboot Toolkit, E2B)

     #4               67106816        4294965240             2.0 TiB        0700               NTFS  (W10ToGO.vhdx )

     #5          4294967288         2516582400             1.7 TiB        0700               NTFS

 
Is there any possibility that either of GRUB2, CLOVER and SYSLINUX will BIOS boot from above partition layout in 4TB ? 
 
If possible I will create two MBRs as BACKUP corresponding to GPT and HYBRID partition scheme of the 4TB USB-HDD in such a way so that partition #3, #4 and #5 will be common and shared by both MBRs as follows.  
 

GPTMBR.mbr -> will have Bootable Protective MBR (0x80) along with all above partitions from #1 to #5

HBDMBR.mbr -> will have Protective MBR (EE00) , #3 (Active) , #4 partitions (also #5 if possible ?)

 

But it will be interesting to see HOW GRUB2, Clover and Syslinux will work in 4TB USB-HDD ? Currently Grub4dos is working using Wonko method. Will chainloading of GRUB2 and CLOVER bootloaders work from Grub4dos / Syslinux so that AIOBOOT and Multiboot Toolkit boot within from 4TB USB-HDD in BIOS mode ?   



#9 Wonko the Sane

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Posted 5 days ago

Well Wonko's method - half-@§§ed as it may seem, besides working, has a "method".

 

To the MBR it is written a modified MBR booting code that chainloads the (fixed in size and location) partition on 63-2047 inside which there is a bootsector that loads grldr, without modifying in any way the "protective" MBR partition entry.

 

The same approach (without using a partition) is used by the UMBR: to the MBR is written a special code that chainloads the grldr directly by its extents on disk (which are hardcoded at "install" time) but that allows also to chainload other targets (such as a bootsector of a partition).

 

The Syslinux approach (from what is written on the page you linked to):

 http://www.lightofda...cgi/BIOSBootGPT

is more evoluted, but basically it writes to the MBR a special code that chainloads a given (GPT) partition (that seemingly is marked by a "BIOS bootable flag") bootsector which itself loads the ldlinux.

 

ALL THESE do not modify the "protective" MBR partition entry.

As a matter of fact the WHOLE point of these approaches is to NOT modify the "protective" MBR partition entry.

 

It is possible (again according to the given page) that some peculiar BIOSes may inspect the MBR partition table for one partition marked "active" and refuse to execute the MBR code if ot is not found.

In these cases (if actually needed) and in these cases ONLY it might be necessary to set the  "protective" MBR entry to active, which is a VIOLATION of the EFI/UEFI specification (though probably it wont' affect anything in "normal" UEFI booting).

 

There are other variations of BIOS booting GPT disks that are defined as "Hybrid", they ALL represent a VIOLATION of the EFI/UEFI specifications and more generally are prone to give issues with this or that tool or OS: 

http://www.rodsbooks...isk/hybrid.html

 

The Syslinux approach seemingly follows (at least partially) the already mentioned/discussed document by Rob Elliot that is listed in the above page at the end in a part titled:

The Other Hybrid MBR

 

In any case the three mentioned methods, can all go under the header of "The Other Hybrid MBR", as they are intentionally designed to NOT modify in any way the MBR "protective" partition entry, and as such they won't (shouldn't) cause any of the issues connected with "normal" Hybrid MBR.

 

SInce they all use the same approach (i.e. write to the otherwise blank/00s bytes 0-440 of the MBR a "special" code) they are mutually exclusive.

And since there is NO other way to boot from BIOS if not to write the bytes 0-440 f the MBR some executable code, ALL OTHER possible methods are as well mutually exclusive, you can have ONE and ONLY ONE of such methods.

 

The Syslinux approach seems the most "flexible" one, as - provided it works - seemingly chainloads the bootsector of the GPT partition (that may be in itself chainload *something else* than ldlinux), still one of the two available grub4dos loading ones seem to me the best choice because from grub4dos it is possible to chainload *almost* anything.

 

:duff:

Wonko 


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#10 devdevadev

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Posted 5 days ago

To the MBR it is written a modified MBR booting code that chainloads the (fixed in size and location) partition on 63-2047 inside which there is a bootsector that loads grldr, without modifying in any way the "protective" MBR partition entry. 

It seems if we add a partition covering the range 63-2047 to the GPT partition table, using following command Sequence then it become more close to The Other Hybrid MBR .

gdisk32 \\.\PhysicaldriveN
x
l
1
m
n
1
63
2047
EF01
p
v
w

The same approach (without using a partition) is used by the UMBR: to the MBR is written a special code that chainloads the grldr directly by its extents on disk (which are hardcoded at "install" time) but that allows also to chainload other targets (such as a bootsector of a partition).

What exact umbr code is used to chainload bootsector of a GPT partition ? Please describe it here in detail.   

 

It is possible (again according to the given page) that some peculiar BIOSes may inspect the MBR partition table for one partition marked "active" and refuse to execute the MBR code if ot is not found.

In these cases (if actually needed) and in these cases ONLY it might be necessary to set the  "protective" MBR entry to active, which is a VIOLATION of the EFI/UEFI specification (though probably it wont' affect anything in "normal" UEFI booting).

Sony VAIO VPCEB3TFX looks similar case with peculiar BIOS in which we are currently testing Syslinux approach from 4TB GPT USB-HDD. It's why I am looking for a Windows command line tool which will allow me to mark Protective MBR of GPT disk as bootable instead of using linux fdisk. 

 

The Syslinux approach seems the most "flexible" one, as - provided it works - seemingly chainloads the bootsector of the GPT partition (that may be in itself chainload *something else* than ldlinux), still one of the two available grub4dos loading ones seem to me the best choice because from grub4dos it is possible to chainload *almost* anything.

Can we not install 'Grub4dos/GRUB2' boot code in VBR of GPT partition instead of using Syslinux boot code so that it will directly chainload 'grldr/g2ldr' instead if ldlinux by any way.....if possible ? Please......

 

Regads....



#11 Wonko the Sane

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Posted 5 days ago

It seems if we add a partition covering the range 63-2047 to the GPT partition table, using following command Sequence then it become more close to The Other Hybrid MBR .

NO, NO, NO!  :frusty:

 

The small code in the MBR (trust me I actually wrote it half-@§§edly modified it from some Syslinux original code) couldn't care less whether the range 63-2047 is mapped in the GPT partition table (or in the MBR partition table or anywhere else). and definitely uses not the (otherwise nice) GPT partition flags.

 

 

 

What exact umbr code is used to chainload bootsector of a GPT partition ? Please describe it here in detail.   

I will pretend you did not make this question (to maintain my sanity).

Anyway the UMBR can (will) also write a MBR code capable of chainloading the VBR of a given partition, so (see below) it could be the alternative you are looking for. (though I still have to understand WHY exactly, since you have the thingy already working with my half-@§§ed approach, you are wanting to deal with these things) 

 

 

Sony VAIO VPCEB3TFX looks similar case with peculiar BIOS in which we are currently testing Syslinux approach from 4TB GPT USB-HDD. It's why I am looking for a Windows command line tool which will allow me to mark Protective MBR of GPT disk as bootable instead of using linux fdisk. 

 

Which is EXACTLY what I provided you.

http://reboot.pro/to...ndows/?p=208468

 

The mission is now to insert after these two lines:





dsfo \\.\Physicaldrive3 0 512 GPTMBR.mbr
dsfi GPTMBR.mbr 0 440 gptmbr.bin

and before this line:





dsfi \\.\Physicaldrive3 0 512 GPTMBR.mbr

a hexalter command capable of changing the byte at 0x1BE in gptmbr.bin from 0x00 to 0x80.

 

How hard can it be? :dubbio:

 

Can we not install 'Grub4dos/GRUB2' boot code in VBR of GPT partition instead of using Syslinux boot code so that it will directly chainload 'grldr/g2ldr' instead if ldlinux by any way.....if possible ? 

Sure we can.

 

Will it work?

No idea.

It is entirely possible that the VBR boot code (unless a somehow "special" boot code is used) may actually *need* an actual partition (I doubt it, but who knows).

As said above, the UMBR can do the chainloading of a bootsector.

 

:duff:

Wonko

 

P.S.: also read this:

 

http://reboot.pro/to...o-gpt/?p=194493

 

AND find here a small batch capable of writing a PBR/bootsector for FAT 32 starting at LBA 2048:

http://reboot.pro/to...o-gpt/?p=192974

 

 

::mkgrldrGPT.cmd Small batch by jaclaz to allow booting to
::grub4dos grldr placed on first partition of a GPT disk
::the partition NEEDS to be EFIsys, formatted as FAT32
::AND start at LBA 2048

you can replace the FAT32grldr.bss with *any* bootsector (as long as it is a FAT32 one)


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#12 devdevadev

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Posted 4 days ago

How hard can it be? :dubbio:

Is it correct way to use it ? Can I proceed with following code ?

hexalter GPTMBR.mbr 0x1BE=0x80


#13 Wonko the Sane

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Posted 4 days ago


Is it correct way to use it ? Can I proceed with following code ?
hexalter GPTMBR.mbr 0x1BE=0x80

Sure.

Why not?

 

:duff:

Wonko







Also tagged with one or more of these keywords: bootable protective mbr, gptmbr.bin, syslinux, gpt, hybrid, grub2

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