Mysterious issue causing backup volume to get unmounted (Ubuntu 22)

I am sorry but I got a temp health issue preventing me to type long, I already explained this weird issue happening with the btrfs partition which was pretty much formatted/dedicated to UrBackup. Pasting from Reddit/btrfs:

I use an external USB drive which passed every kind of test possible even including the awfully long badblocks rw test. Full smart test doesn’t show anything wrong either.

It is mainly used for backing up 2 Windows 10 machines via UrBackup and occasional old fashion full clonezilla images.

Right at 1 AM (it is a restaurant), urbackup starts to backup PCs. As a precaution I even disabled automount as it may unmount idle devices. After 2-3 days when the system warns me less than 1GB left on root folder, I notice it got somehow unmounted and urbackup (or OS?) decided to use /backups as an ordinary directory.

It is a 22.04 LTS with HWE kernel 6.2.

I type ‘sudo unmount /backups’ it says: “umount: /backups: not mounted.”

OK, I say ‘sudo mount /backups’ and the result is: “mount: /backups: mount(2) system call failed: File exists.”

dmesg includes: " BTRFS warning: duplicate device /dev/sdc2 devid 1 generation 593 scanned by mount (2249065)"

I do have secondary and third backups in other services such as duplicati/google drive but in case of an emergency it will take hours to get systems up and running.

Last time, I kept using the -f switch which is never recommended for both mounting/unmounting and it worked eventually leading to massive metadata corruption. This is the second time I formatted the drive with BTRFS and used “dup” profile to increase my chances of data recovery.