Sorcha - I was asking similar questions about Degfrag a few months back. I had just had a defrag which took about 13 hours and was wondering why it kept stopping and starting from scratch.
I found JohnInKansas This Thread!!!!!!!! gave very useful advice, as did various other 'catters.
The main point was that once the major defrag has been completed, there should be a big sector of your disc sorted out, and new information writes to the remaining bit. As a result, most of the new defragmentation is confined to a limited portion, so if you then defrag it is working on a much smaller (and therefore quicker) job.
Is that right - John?
Anyway, since that time any defrags I have done take a considerably shorter time. It still stops and re-writes itself, but this doesn't hold things up by much.
Quack!
GtD.