I have a problem where GRUB 2.12 fails to access ext4 filesystem, but
only during MBR root, not during UEFI boot.
I've installed GRUB 2.12 on a USB SSD with ISO-file in ext4 filesystem
(exact install steps listed below).
When I boot this in UEFI mode it works every time, but when I boot in
MBR mode GRUB fails to access ext4 filesystem and
computer reboots.
USB SSD: Samsung Portable SSD T5 (1 TB)
========== COMPUTER 1 ==========
Main computer where I install GRUB to USB SSD.
OS: Debian 13
$ uname -a
Linux box 6.12.63+deb13-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.12.63-1
(2025-12-30) x86_64 GNU/Linux
$ grub-install -V
grub-install (GRUB) 2.12-9
========== COMPUTER 2 ==========
Old computer where I try to boot USB SSD.
Computer : unknown Fujitsu model
BIOS Vendor : American Megatrends
Version : 2.02.1205
Core Version : 4.6.4.0
Compliency : UEFI 2.0
Project Version : 1AOQI 0.12 x64
Build Date and Time: 02/09/2012 11:54:52
Secure boot is disabled.
========== GRUB INSTALL ==========
; 1 GiB FAT32 EFI System Partition
; 1 MiB none BIOS Boot Partition
; 750 GiB ext4 "Images"
DEV=/dev/sdc
$ sudo sgdisk -Z $DEV
$ sudo sgdisk -I -o -n 1:0:+1G -t 1:ef00 -n 2:0:+1M -t 2:ef02 -n
3:0:+750G -t 3:8300 $DEV
$ sync
$ sudo partprobe
$ sudo mkfs.vfat -F 32 -s 8 ${DEV}1
$ sudo mkfs.ext4 -L Images -E root_owner=1000:1000 -O
^metadata_csum_seed ${DEV}3
$ sudo mkdir -p /mnt/temp
$ sudo mount ${DEV}1 /mnt/temp
$ sudo mkdir /mnt/temp/boot
$ sudo grub-install --target=i386-pc --recheck
--boot-directory=/mnt/temp/boot $DEV
$ sudo grub-install --target=x86_64-efi --removable
--boot-directory=/mnt/temp/boot --efi-directory=/mnt/temp
$ sudo vim /mnt/temp/boot/grub/grub.cfg
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
menuentry "Images" {
insmod configfile
insmod search_fs_uuid
search --set --label Images
configfile /grub.cfg
}
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
$ sudo umount /mnt/temp
$ sync
* copy debian-live-13.3.0-amd64-xfce.iso to ext4 partition
* create grub.cfg in ext4 partition:
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
menuentry "Xfce" {
insmod linux
insmod loopback
set iso="/debian-live-13.3.0-amd64-xfce.iso"
loopback loop ${iso}
linux (loop)/live/vmlinuz boot=live components findiso=${iso}
initrd (loop)/live/initrd.img
}
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
========== TESTING RESULTS =========
When booting via UEFI, everything works and I always boot to Xfce.
When booting via MBR, selecting "Images" in the first menu causes the
computer to reboot everytime. I never get to the second menu with the
"Xfce" item.
When using the command-line with the following steps, it sometimes
works and I boot to Xfce, and sometimes it fails at `search` or
`linux` commands.
search --set --label Images
loopback loop /debian-live-13.3.0-amd64-xfce.iso
linux (loop)/live/vmlinuz boot=live components
findiso=/debian-live-13.3.0-amd64-xfce.iso
initrd (loop)/live/initrd.img
boot
Any suggestions on what the problem is and how I could get MBR boot to work?
--
Markus Laire
