Passport for Mac Suddenly Dead: "An Error Occurred While Creating the Backup Folder"

My Passport for Mac worked fine for 12 days, and today it suddenly died. I now get error messages on my Mac saying only “An error occurred while creating the backup folder.” When I click on the Passport icon, it looks like it’s empty. Under “Disk Utilities,” it says the drive is 1/8 full. Is the unit worthless now? (It was never dropped or physically messed with.)

UPDATE: I ejected the Passport, re-mounted it, and it backed up fine, and all the previous backups were in there. Strange.

(edit for typo)