So says the message in Disk Utility. This a 3TB Seagate GoFlex FireWire with a one-TB partition for general use and a two-TB partition for Time Machine backups. 27-in. late model iMac with latest Mac OS, etc. Time Machine began reporting back-up failures after 24 April. I turned off TM because I have other backups of my internal drive (SuperDuper) and I didn't have time to deal with the issue earlier. The TM partition passed the First Aid routine with no errors. Since nothing else worked to get Time Machine working on this partition I decided to start anew with it by erasing it. The error message tells me that ³Volume Eras failed with the error: Mediakit reports not enough space on device for requested operation.² In fact, the disk has 1.25 TB used and 745 GB available. It seems to me that the disk ought to be erasable. Ideas welcome. TIA!