Mmm..
It really shouldn't be a fragmentation issue.

I wonder if that operating system/version can be tuned to handle the recording load better.

I know this isn't helpful for your specific setup, but on the Myth box here, we can have six ATSC streams (4HD + 2SD) plus two NTSC streams all recording simultaneously, with two commercial flaggers running, and one playback running. Glitch free.

The storage is a pair of 750GB SATA drives in RAID0, formatted with XFS. A third drive holds the system and database.

If your O/S allows separating the journal file, then putting the journal on a separate/dedicated drive might help considerably. We haven't needed to do that here yet, though.

Cheers