My internal backup drive is mounted, but when I try to start a backup, or recover missing files, I get the above message. It also says "Waiting for [device] to become available" (translated from French).
This similar question was not useful. I tried creating a folder called "deja-dup" on the backup device, but nothing changed. I have also tried unmounting and remounting the drive.
I had the drive itself (not the mount point) set as the backup location, but for some reason the drive just isn't recognized as connected. Here's what I did:
/mnt/Earth/Backup/Ubuntu home