Hard drives can fail. That is why you have copies of important files and memories on at least two drives, but preferably more. Then, when a drive fails, you do not loose your data.
You used the drive for 10 years. That is a long time for a hard drive to remain operational. Windows 10 does not anymore recognize it. That, now, also Ubuntu does not recognize it ...
CIFS is a virtual filesystem that creates a "view" of the remote share with a set of permissions specific to the client - not the server. You cannot chown / chmod a virtual filesystem. And cifs has no visibility to permissions on the server.
By default a cifs mount will create a "view" that has owner = root ( unless you specify uid / gid ...
You can tell the mount point in /etc/fstab.
So you would set the name, and your partition would always mount with the same name.
The header of the file is reasonably explanatory
$ cat /etc/fstab
# /etc/fstab: static file system information.
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust ...
The mount command attaches a filesystem to a directory. You can actually attach it at any directory and you should create one for it. The /mnt directory is a frequent target of such mounts and is an empty directory created by default on most unix systems specifically for that purpose. You can mount multiple things on /mnt, even at the same time, but if ...
As I have not found a solution to this question using systemd (Ubuntu 20.04, s3ql 3.8.0) easily I will post my solution here:
Description=mount s3ql filesystem
ExecStart=mount.s3ql --systemd --fg <other options> <storage url> <mount point>