SATA Drive There...and then Not

I have an older ASUS M/B. It's an A7N8X-E Deluxe. About a month ago, I began having an issue with my secondary (D: ) drive. The drive would periodically disappear from Vista.

At the time my drive setup was an 80GB WD SATA drive as my C: drive and an older ATA 20GB (Seagte I believe) as my D:. I assumed the the older drive must be dying, so I replaced it with another 80GB WD SATA drive. I left the 20GB installed, and it's been fine since then.

My C: is the new 80GB SATA, my D: drive is now the original 80GB SATA, E: is now my original 20GB ATA.

Now the problem has recurred with my D: drive. It disappears from Windows. BIOS (actually the RAID software) still sees it fine, but it's inaccessible. It's important to realize that it does work usually, but sometimes it just isn't there. My older drive has not shown any signs of disappearing.

The only sure way to bring the drive back is to actually shut down power to the M/B and then reboot. This has always worked, but I don't know if it will continue to do so.

It's probably also important to mention that I re-installed Vista on the new drive and started from scratch. I can't see it being an O/S issue.

It's curious that the issue has been with my D: drive both times although they are different sizes, different manufacturers and different technologies...

I'm at a loss, but it may be time for a new M/B although this one still suits my needs perfectly.

Any ideas...?