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I didn't have one handy so decided to copy stuff off and try reformatting. Initially it seemed to be split into two partitions and I could only format one, regardless of attempts to wipe, extend, etc.
Eventually ended up using Diskpart to Clean it and then creating a partition with Minitool Partition Wizard. So far as I could gather the problem was something to do with a missing GUID Partition Table but works fine now.
Does it work if you boot up with a live CD?