My original question "Defrag won't work" was solved by disabling NAV. This seemed to allow Defrag to run freely without restarting (ie there was not one "drive's contents changed - restarting" message).
Perhaps because of recent uncompleted runs, Defrag breezed through until about 88% complete when it seemed to stick on a couple of clusters. At this point it had been running about 15 minutes and it took another 15 minutes before these clusters showed as optimized. For the next two hours (!) it seemed to pause on a cluster or two every line and took between 10 and 15 minutes to 'resolve' these, show them as optimized and move on only to immediately 'stall' on another cluster. During this time the % scale varied between 85% and 91% complete and the rhythm of the drive noises seemed constant. I eventually shut it down, worried in case it was putting strain on the drive/processor. What's wrong?
Responses as comments please ...... and no, its not a screensaver - none are running.