I've been building my own rigs for 20 yrs n never had problems like these....
I have a raid 1 (mirror) and I'm currently recovering from my 2nd HD failure on port 1.
On port 1, have had Samsung Spinpoint F1 (HD753LJ) drives fail twice - one after 18 months, latest one after 10 weeks. Wondering if this is a HD problem or something that points to MoBo issues? I always buy n fit my HD's in pairs, so the replacement drive I have just fitted is only a few S/N no's after this last drive that lasted 10 weeks!
If the drive isn't specifically designed for use with a RAID controller then it can be declared "dead" by the controller when it takes to long to recover from an unreadable sector. RAID-ready drives use TLER (Time-Limited Error Recovery) to avoid this issue.
Double check everything you can to do with your PSU. I had this happen once, but got through a whole 8 DRIVES before finding out it was my PSU frying them. They would start to click, and then within days they would fail. Nothing else had any issues on my system, but drives - it ate them up for breakfast.
Besides that, not really sure what it could be other than a faulty motherboard or just unlucky.
Oh, and sminlal might have hit it on the head. I don't know much about raid and that sounds reasonable! :]