
It appears that if someone neglects putting media in the drive when MLS is about to do its backup, MLS will default to saving the backup file to the local hard drive. I think that this is probably the right thing for MLS to do because it at least makes sure that a backup got created. The problem is that from that point forward, until told otherwise, by default, MLS will continue backing up to the hard drive and not prompt for the insertion of the removable media for subsequent backups.
On further investigation, I found other units who had gotten into the same situation and were no longer backing up to removable media. In each case they had assumed that this was a new "feature" in MLS.
A change to MLS might be in order. In the event that the backup file is about to be written to non-removable media, the user should be warned that in the event of a hard drive failure, their backup could be lost. Then encourage them to save the backup elsewhere.