@Mad_Mattho The deskop version while not supported should work. I think the issue here is something else though.
I think the issue is that the mount is mounted with some strange permissions. Did you use gnome to mount the disk? I think gnome mounts the external disk as the ubuntu user you logged in as. This would prevent the yellowtent user (that cloudron runs as) from accessing that mountpoint.
So, can you please try this:
Unmount the disk in gome Mount the disk in the Backups view as an ext4 disk . You can use blkid or lsblk to get the uuid of the disk . Then, /dev/disk/by-uuid/<uuid> is the Disk path.