Pausing in the middle of a defrag, allows the "priority" app to then start writing to the disk... quite possibly into the middle of the "new" destination area for the defrag.. requiring the whole thing to be done almost from scratch again.

And with files in the 5-50GB range (as on our PVR here), any attempt to defrag would take so long that it could never really hope to complete before the next use of the PVR.

Cheers