The error I got was "failed to shutdown Logical Volume Manager (LVM)" with the only additional qualification that umount had an error. Why or what to do next wasn't addressed.
In looking at a forum thread on the matter, it suggested that perhaps this error message wasn't correct due to a bug in Filezilla (though I was using a version beyond the one specificed with the known problem).
It wasn't clear how to 'shutdown the LVM' myself, or what that actually meant. LVM stands for Logical Volume Manager, which can pool devices, make resizable volumes, handle data encryption, taking snapshots, etc.
Here was my solution that worked for me.
What I suspect is going on is that the destination is actually a logical volume exposed by LVM, and so this management process is still likely happening at the time you're trying to do a restore.
Using GParted, repartition the destination drive, leaving the device unformatted, and then do the clone to restore.
Part of me twitches, because this is indeed a destructive operation (then again, so is doing a restore); the hope being if a restore won't start, you at least have the old contents around. Not by this method you won't. Unless you take an additional backup of the data you're about to stomp on.