I think that as soon as a backup is run, either manually or scheduled, the disk is unmounted automatically.
The dar2 contrib mounts the disk when required, writes the backup to it, and the unmounts it (I guess to keep the data on the backup disk as safe as possible.
To see the backup you have to run the command mount -a which mounts all your file systems again. You'll see the results of the backup in the expected location.
The older version of the contrib, IIRC, had the option of unmounting after backups or not. It's probably best to have it unmounted anyway.
I hope this helps.
Steve