CompTIA Linux+ (XK0-006)TroubleshootingMedium
After a kernel update, a server fails to boot and drops to a `grub rescue>` prompt displaying "error: unknown filesystem". What is the most likely cause?
- AThe /etc/fstab file contains an incorrect mount option for /var
- BA required kernel module is missing from the initramfs image
- CGRUB cannot locate the /boot partition due to a changed UUID or corrupted grub.cfg
- DSELinux is set to enforcing mode and blocking the boot process
Show answer & explanationAnswer & explanation
Correct answer: C. GRUB cannot locate the /boot partition due to a changed UUID or corrupted grub.cfg
The 'unknown filesystem' error at the grub rescue prompt means GRUB's core image cannot identify or locate the filesystem containing its configuration and boot files, typically due to a UUID mismatch, disk reordering, or corrupted grub.cfg — this occurs before the kernel or initramfs is even loaded.
Why the other options are wrong
- A. fstab errors affect mounting after the kernel loads, not the GRUB stage itself.
- B. Missing initramfs modules cause a kernel panic or dracut emergency shell, not a grub rescue prompt.
- D. SELinux enforcement issues occur after the kernel and init system start, not at the GRUB prompt.
GRUB Rescue Prompt
The grub rescue> prompt with 'unknown filesystem' indicates GRUB cannot locate its boot partition, usually from UUID changes, disk reordering, or corrupted grub.cfg.
- Occurs before OS/kernel loads
- Fix by locating boot partition (ls, set root=) and reinstalling GRUB
- grub-install and update-grub/grub2-mkconfig regenerate configuration
Memory trick: GRUB rescue = lost map before the journey even starts