When I was looking into fragmentation issues possibly affecting my PVR recording quality (it does), I noticed that writing files serially to the drive didn't necessarily produce a nice compact unfragmented disk.

I ended up installing defragmenting software that monitors the drives and defragments whenever they're not being used. Along with keeping a good amount of free space on each drive, I no longer seem to have any issues when writing out three or four HD recordings at one time.