I was in a similar situation recently with a CD Rewriter and was told that I was deemed to have accepted the goods for the following reason described in the Sale of Goods Act:
"By keeping them for longer than a reasonable time without telling the seller that you have rejected them. There is no time specified in the Act."
The retailer said they considered 28 days to be a reasonable time and therefore I was only entitled to a repair, which could take up to six weeks.
I bought a replacement and sent the original drive back (at my expense). I'm calling their bluff out of principle. What's the chance that the manufacturer is going to repair a drive that's only worth £12?