SystemUIServer[163]: Error getting disk for path /Volumes/1TB Backup Disk: No underlying device was found (for example, AFP network volumes are not supported)

I've looked for this solution for at least an hour before posting. Here's my situation: I back up to an external drive connected to an Airport Extreme. It takes Time Machine at least five minutes while "Looking for backup disk". After that, it takes a while preparing the backup, and then the backup itself is painfully slow (almost an hour for a GB. That is slow, right?). I get the above error literally once per minute in the Console, WHILE it's actively backing up.

Any help is appreciated. I can post more details if that's needed.