Hi,

could you check gnu/bootloader/grub.scm install-grub (and maybe install-grub-efi) ?
They are build-side code so you can just use (write "foo\n") to add debugging messages.
Interesting would be DEVICE, MOUNT-POINT.

FWIW, I successfully built a new installation iso image from guix master (and booted it) a few days ago.

I still have that image and file(1) says:

>guix-master-i686.iso: DOS/MBR boot sector; GRand Unified Bootloader, stage1 version 0x79, boot drive 0xbb, stage2 address 0x8e70, 1st sector stage2 0xb8db31c3, stage2 segment 0x201; partition 2 : ID=0xef, start-CHS (0x3ff,63,32), end-CHS (0x3ff,63,32), startsector 2483356, 5760 sectors