Create legacy bios/uefi dual boot usb stick

[John Frankish]

Hi,

I've been trying, without success, to create a legacy bios/uefi dual boot
usb stick.

If I partition the usb stick using gdisk, like this:


$ sudo gdisk /dev/sdc
...
Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048         2097152   1023.0 MiB  EF00  EFI System
   2         2099200         2103295   2.0 MiB     EF02  BIOS boot partition
   3         2103296        61013982   28.1 GiB    8300  Linux filesystem

$ sudo mkfs.vfat /dev/sdc1
$ sudo mkfs.ext4 /dev/sdc3

$ sudo mount /dev/sdc1
$ sudo x86_64-grub-install --target=x86_64-efi
--boot-directory=/mnt/sdc1/EFI/BOOT --efi-directory=/mnt/sdc1 --removable

$ sudo mount /dev/sdc3
$ sudo x86_64-grub-install --target=i386-pc --boot-directory=/mnt/sdc3/boot
/dev/sdc2

..the last line fails with an error message that I forgot to note down.

If I then try:

$ sudo x86_64-grub-install --target=i386-pc --boot-directory=/mnt/sdc3/boot
/dev/sdc

..there is no error message, uefi boot works fine, but legacy bios boot
fails with an "invalid partition table" error

I assume I'm making a mistake somewhere, but where?

Regards
John