How to mount one of multiple disks to a specific location in fstab

Question

I have a small number of removable hard drives. At any one time, one of them will be mounted to /backup except while changing drives. I swap the drive periodically. That is I have 4 hard drives and I rotate them.

Currently I manually mount / unmount the drive. But there are times when this machine is turned off and worse, sometimes it gets turned off without my knowledge. The daily backup script will fail if a drive isn't mounted.

The drives don't currently share a UUID or Label. I can't garuntee that the disk will always be available on /dev/sdb1. Is there a good way to mount one of a number of drives automatically from /etc/fstab when I just don't know which drive will be inserted?

---

Note this is a linux (debian) system without a monitor or keyboard. Drives are currently manually mounted / unmounted over ssh after I plug / before I unplug.

Answer 1

After coming back to this question a long time later I've realised the solution is actually the same as optionally mounting a drive in /etc/fstab. This is discussed here https://wiki.archlinux.org/index.php/fstab#External_devices

In short my solution is to simply have two almost identical entries mounting to the same place. Eg:

UUID=cd49ca72-db24-47ba-b3bc-f0ba8e290599 /backup    ext4    nofail,x-systemd.device-timeout=1  0       0
UUID=d28c6d3a-461e-4d7d-8737-40a56e8f384a /backup    ext4    nofail,x-systemd.device-timeout=1  0       0

As long as only one of them is plugged in when the system boots, the other will "silently" timeout after a 1 second. So whichever is plugged in will get mounted, and the other entry will not trip up the boot process.

Note: Only use this solution if toy are confident that only one of the drives will be mounted at a time.

Answer 2

What about if you generate a new uuid to all those drives if you will not plug them all at once?

Generate a new uuid with the uuid command(on some systems, it is the uuidgen command. Example:

# uuid
30dcffac-1187-11e8-8338-0050569774ad

Check your current UUID, and if you want, write it down anywhere:

# vol_id /dev/sdb1 | grep UUID
ID_FS_UUID=224990ae-1187-11e8-ba4c-0050569774ad
ID_FS_UUID_ENC=224990ae-1187-11e8-ba4c-0050569774ad

Change the UUID using tune2fs. Do this once per external drive you have:

# tune2fs /dev/sdb1 -U 30dcffac-1187-11e8-8338-0050569774ad
tune2fs 1.41.3 (12-Oct-2008)

Mount it using the new generated UUID at fstab. Example:

UUID=30dcffac-1187-11e8-8338-0050569774ad  /backup  ext4  defaults    0       0

This way, mount -a should be enough to mount the drives on-the-fly, and if the computer is restarted it shall mount it automatically the partition with UUID 30dcffac-1187-11e8-8338-0050569774ad. And as i have not tested this on a physical machine, i don't know what are the consequences of inserting 2 of those disks at the same time and trying to mount them.

If the disks are GPT format, PARTUUID should be used at fstab.