Ubuntu keeps randomly switching disks

Question

It took me a while to notice, but here's what happens. I have 2 disks on my computer.

On startup, Ubuntu's Disk Utility tells me one is sda and the other sdb. On restart it switches them and on another restart, switches again. It's impossible for me to set my /etc/fstab with the OS randomly switching my disks. I am not changing any settings in BIOS.

How can I make this stop?

Answer 1

Use the device mapper. Instead of mounting, say, /dev/sda1, mount by ID. This is how the respective line from fstab would look like:

UUID=xxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxx  / ext3 relatime,error=remount-ro  0  1

You can find out what the id's are with blkid:

/dev/sda3: UUID="xxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxx" TYPE="ext3"

The above UUID is what you should use in the fstab entry above.

From the fstab man page:

Instead of giving the device explicitly, one may indicate the (ext2 or xfs) filesystem that is to be mounted by its UUID or volume label (cf. e2label(8) or xfs_admin(8)), writing LABEL= or UUID=, e.g., 'LABEL=Boot' or 'UUID=3e6be9de-8139-11d1-9106-a43f08d823a6'. This will make the system more robust: adding or removing a SCSI disk changes the disk device name but not the filesystem volume label.

Answer 2

This is why Grub uses UUID-s instead of old-fashion /dev/sd* reference. The UUID is unique, and it doesn't matter in which order is seeing the OS the disks. So, update your /etc/fstab to use UUID-s, and you sould not worry about it anymore. (More info here.)

The reason for switching should be a BIOS error (this needs some more investigation).