I've discovered that my raid hiccups when recording 3 or more shows (I should've spent the extra $150 on the raid card that had a dedicated processor! damn), so I'll be eventually getting a dedicated 2 TB for mythtv recordings.
Strange, because years ago I used software RAID (the Windows software RAID feature) for a recording drive using "green" drives, and I did a "stress test". I quoted stress test because it hardly registered as far as queued requests go on the drive using perfmon. And that was with 5 TV recordings being recorded and 4 different recorded shows being viewed (3 on "clients", and one on the server).
I did the same test with all that activity going on with one physical drive (no RAID) and it still had no problems keeping up.
HTPC drive access platters involve a lot of sequential reading and writing. This is something that mechanical drives are very good at.
Your concurrent recording problem must be a software (RAID driver?) issue.
BTW, with modern processors I highly recommend software RAID; specifically the one built into Windows. I've found to to be extremely reliable, and the performance delta between it and "real" hardware raid is practically non-existent because modern processors have gotten a lot faster, where the performance increase of mechanical drives has hardly budged in the mean time. Especially considering that most computers seem to have "spare" processor power in the form of idle cores.
On the other hand, software raid that depends on drivers from companies like HighPoint, Adaptec, SYBA, or LSI... I've seen more problems with those. I think the reason is that fewer people use them and the software development processes those companies use have less rigor than what goes on at Microsoft.