Thanks MrRon, I would have just put it down to one of those things if it had happened just once, but it happened on 2 drives from different manufacturers, one an 80 GB drive and one 60 GB. I defragged them both on the same machine and without rebooting it in between. I did tick the option to defrag all the files that had been identified as needing a defrag and there were no error messages from either run. In fact everyything looked perfect until I tried to reboot each drive and then the blood drained from me as realisation dawned - I accept that I should have been a bit more cautious but given my excellent experience with CCleaner over the years I wasn't expecting any issues defragging individual files. If I remember correctly some of the file names that came up were system files names like System Information - I can't remember them exactly but they didn't look like file names that you normally see. I have reformatted one of the drives (the 80GB one) and its now running the system I'm writing this from but the other drive is still in the state it was left in - accessible, but non-bootable. If you have some tool that will extract the boot records for you to look at you're more than welcome to contact me via my email and I'll run the tool for you and send you the output if it will help you.