I've run across it before. Never did track down what caused it. However, if I recall, a reboot generally fixed it.
I suspect it has something to do with the temp files that are created when you've been recording for awhile - however that's just a WAG. My recollection is that after I rebooted the problem went away.
Oh, and btw, I defrag pretty regularly, so that wasn't it (at least in my case).