Two very different/separate questions.
#1 Sure it is possible to change the CODE in a MBR without changing the DATA in it, but this should not in any way affect a "data only" disk drive.
If you prefer when booting the CODE inside the MBR is read/executed (by the BIOS) whilst in a "data only" drive ony the data inside it (Disk Signature and Partition Table) is used.
#2 there is something that doesn't sound right with your report (or maybe it is only confusing and I cannot get it fully right).
![:unsure:](http://reboot.pro/public/style_emoticons/default/unsure.png)
Anyway the sector size CANNOT normally be changed via software (and SHOULD NOT) be changed.
The disks are either NOT "identical" OR you are testing them differently OR *somehow* one of the two changed its behaviour (which I strongly doubt, but which still it is possible, at least in theory).
There are some USB "external cases" or "SATA-USB converters" that behave strangely, i.e. automagically convert 512 bytes devices to 4096 or viceversa, (whilst when directly SATA connected they use "native" sector size) if the disk drive is of the "new" (stupid) AF (or Advanced Format) kind.
Your drive(s) is/are seemingly:
https://community.wd...rnally/15580/10
AF (and thus use 4 kb sectors internally), it is entirely possible (though as said not "probable") that something in the disk drive firmware "recognizes" a particular partitioning pattern (or *whatever*) and turns off and on the internal translation from 4096 bytes/sector to 512 bytes sector, but it is more probable that the issue is with the USB to SATA bridge.
Normally an AF drive uses internally 4Kb sectors but exposes a translated 512 bytes geometry, we had a very similar issue here:
http://www.msfn.org/...ent-interfaces/
and now that I am re-checking that thread, I see that it is the EXACT SAME disk drive model
![:w00t:](http://reboot.pro/public/style_emoticons/default/w00t.gif)
![:ph34r:](http://reboot.pro/public/style_emoticons/default/scared9.gif)
Please take some time reviewing that thread on MSFN, JFYI at the time I came out with a specific workaround, NOT solution, here:
http://www.msfn.org/...ent-interfaces/
Then, please start a new thread, (the matter is not suited to this generic Bootice thread) here:
http://reboot.pro/fo...rmat-utilities/
and post there a copy of the MBR of the two disks and the oputput of the fsutil command like Dave-H did on MSFN.
Since you have two drives that behave differently, maybe this time we will be able to find an actual solution.
![:duff:](http://reboot.pro/public/style_emoticons/default/duff.gif)
Wonko