OK, just in case anyone else has the same problem. When I tried the trick of removing the power after selecting the drive,it worked! I managed to format the drive successfully but when I plugged my first disk back in (the one that was originally fine) it was now showing 32 meg even though I had previously formatted it with no problems and had loaded about 300 gig of data on it. Um.
Finally had to admit it was a motherboard problem rather than a disk problem so I bit the bullet and bought a new MSI motherboard and replaced the gigabyte one I had in there. Bingo! The capacity restore facility worked first time, my data was intact (phew) and now both disks are functioning perfectly.