Hi, i assume -bootdata:2#p0,e,b.\boot\etfsboot.com#pEF,e,b.\boot\efiboot.img is some shortcut for the options -p, -e, -b as listed in the technet.microsoft.com manual of oscdimg. If so, then i can quite follow the conclusions 1 and 3. (Number 2 is out of my scope.) kyvaith wrote: > Conclusions: > 1. In fact, Windows does not require the UDF file system to boot from EFI Then we probably do not need mkisofs/genisoimage. (But you did not explicitely show a try of mkisofs -udf -b.) > 3. Windows starts up with grub2, if inside the ISO there is no > any El Torito images. http://www.uefi.org/sites/default/files/resources/2_4_Errata_B.pdf 12.3.2 "Partition Discovery" describes precedence of GPT over El Torito over "legacy" MBR. The examples produce neither GPT nor MBR. So maybe El Torito captures the device description and the Microsoft boot loader compensates for this. Or maybe El Torito brings the boot process to the GRUB2 in the ISO, which then fails for its own reasons. (See below, fallback GRUB2 theory.) > OK, Windows boot from GRUB2 menu (bootmgfw.efi), but don't boot from legacy > mkisofs ... -b ... -no-boot ... The -no-boot option is supposed to mark in the El Torito boot catalog the entry of the -b image file as not being usable. This might have the same effect to the firmware as if the entry was not there. Whatever, the main question is how does the firmware find the first GRUB2 file (bootx64.efi, i assume) inside the FAT image file /boot/efiboot.img of the ISO filesystem ? The two first successful mkisofs runs give no clue by El Torito or partition tables. Do you perhaps fall back to the GRUB2 on some other device ? Have a nice day Thomas