As far as i know, it's not possible to create a single USB stick which is 100% compatible with all kind of firmware - Legacy BIOS, x86 UEFI and x64 UEFI. This is mainly due to non standard/custom firmwares used by different device manufacturers. However my own personal experience is that a MBR partitioned FAT32 formatted USB stick is supported on majority of the UEFI and Legacy firmwares. I have read reports about Tablet PC firmwares which will only boot from GPT partitioned disks. If you have to regularly deal with such devices, i would suggest creating another multiboot USB stick which is partitioned under GPT scheme. Even with that, you only need to carry two USB sticks instead of six.
Before you begin, please read http://www.easy2boot...things-to-know/
The steps are :
1. Use DISKPART to clean the current partition table of the USB stick and create an active primary partition which is formatted as FAT32.
Option three of http://www.sevenforu...ive-create.html
2. Copy the entire contents of 64 bit Win10 DaRT disc to the stick. The resulting USB stick will have one "boot.wim" under "Sources" folder and will boot either in Legacy or UEFI mode (at least on most systems).
MBR/CSM mode booting sequence is BIOS-->MBR of the stick--->PBR of active partition-->Bootmgr-->BCD-->WIM
UEFI boot sequence is UEFI-->\EFI\boot\bootx64.efi-->BCD-->WIM (64 bit UEFI)
3. Rename and copy other DaRT boot WIM images to the sources folder on USB stick- ie, rename boot.wim of DaRT 8.1 64 bit to DaRT8.1x64.wim.
4. Use BOOTICE or EasyBCD to add the new renamed wim files to the Boot\BCD BCD store.
5. Add 64 bit based DaRT wim files to the EFI\Microsoft\Boot\BCD bcd store.
The resulting stick will be compatible with most systems -bootable on majority of the desktops and laptop i have tested so far. For rogue UEFI firmware which require GPT disks (won't boot from a MBR disk), you can prepare another USB stick which is GPT partitioned. 32 bit UEFI firmware may also probably boot from the multiboot USB stick if \EFI\boot\bootia32.efi file is present. However i have zero experience with such firmware. If i am not mistaken, 32 bit UEFI firmware is exclusive to Tablet PC and extremely rare to find on either Desktop/Laptop computers. Tablet computers are rare on where i live (you will only find Android tabs).