I recently ran into a similar issue with my Time Machine backup disk. After about 6 days of no backups - I had swapped the disk for my photo library for a media project; I reattached the Time Machine disk and attempted a backup.
Time Machine could not backup to the disk. Running Disk Utility and attempting to Repair the disk ended up returning the "Live file system repair is not supported" message.
After much experimentaion with disk analysis softwares, I came to the realization that the issue might be that the USB disk dock wasn't connected directly to the MacBook Pro - it was daisy-chained through a USB Hub.
Connecting the USB disk dock directly to the MBP and running Disk Utility appears to have resolved the issue. DU ran for about 6 hours and succesfully repaired the disk. Consequently, I have been able to use that Time Machine disk for subsequent backups.