How to mount devices with different UUID that are not present at the same time at the same mount point?

Question

I have external USB HDDs that are used for backing up data and that rotate over months. Only one of the disks is present at the same time.

The disks have different UUIDs (yes, it is possible to clone it, but I would try without).

/etc/fstab looks like

UUID=e4281b1d-44e9-441d-9158-2b6f26beafaf /media/backup ext4 defaults,nofail,x-systemd.device-timeout=1s 0 1
UUID=7e5038f3-e44a-4413-a5ea-ef8e8cdf725c /media/backup ext4 defaults,nofail,x-systemd.device-timeout=1s 0 1

If the only first disk is present it works fine. If only the second disk is present it does not work with sudo mount -a -v or restarting the system.

What really irritates me is that if only the second device is present after a restart, sudo mount -a -v states that the HDD is already mounted, but it does not show up if I use mount.

If I comment the first device in /etc/fstab like

# UUID=e4281b1d-44e9-441d-9158-2b6f26beafaf /media/backup ext4 defaults,nofail,x-systemd.device-timeout=1s 0 1
UUID=7e5038f3-e44a-4413-a5ea-ef8e8cdf725c /media/backup ext4 defaults,nofail,x-systemd.device-timeout=1s 0 1

it works as expected on restart or sudo mount -a -v and mount shows the device as

/dev/sda1 on /media/backup type ext4 (rw,relatime)

I have no clue what the reason could be. Do you have ideas?

I saw this solution that mentions a possible solution with udev, but if possible, I would like to avoid this: Mounting alternate devices (with different mount options) at the same mount point

Answer 1

As a quick workaround for a single backup mount (using any physicall disk) I give them all the same LABEL and use LABEL=xxx in fstab. So there is only ONE entry in fstab.

For tracking pourposes I have an spreadsheet with the UUIDs and the hand written name in the disk to know where and when I mounted the disk and which disk I am really using.

LABEL=my_generic_backup_disk /media/backup ext4 defaults,nofail,x-systemd.device-timeout=1s 0 1