Can't Erase External Drive Partition

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!

 

Methinks that something is wrong with the partition structure of the
disk. I would back up the general use partition to a separate drive
using SuperDuper! and then attempt a reformatting and partitioning of
the drive in question. If that works you could then restore the backed
up partition.