I'm posting this in case it helps someone out in the future.
I have two WD Raptors (74GB) that have been running quite happily in RAID-0 configuration for a few months. However, last Friday I decided to undo the RAID-0 configuration to go back to a C: and D: drive. I had to back up anything important and reinstall XP Pro. Everything went perfectly smoothly and I was back to normal in around 5 hours (all apps/games, e-mail and Internet connectivity) from start to finish.
Last night (Saturday), imagine my horror when I switched my machine on and I got a hard drive failure message!!! I tried switching on and off a few times with no luck. I checked the BIOS for the presence of my two Raptors - it only showed the second Raptor. I was pretty gutted. I opened up my case and unplugged the SATA cable and power cables to both drives. Still the first SATA drive would not show up. I assumed the worst and was planning on ordering another drive (all my essential data was on the working second drive and also backed up onto DVD so things were not as bad as they seemed at the time!).
Anyway, I noticed the Raptors had two power connectors - a SATA power connector and a traditional 4-pin molex connector. I read the label on the top of the drive and it said only ever use one of the power connectors. I had been using the SATA power connector up until then. I unplugged the SATA power connector (my Tagan has 4 SATA power connectors on it). I tried the other three SATA connectors from the PSU but none worked. I then plugged a standard 4-pin molex connector from the PSU to the Raptor, ensuring that I had unplugged the SATA power connector, and the drive burst into life! The SATA power connector on the drive itself appears to be faulty.
Hence, the moral of the story is to check if you have a standard 4-pin molex connector on your SATA drive if you suspect it has failed and to try powering the drive via that (ensuring the SATA power is unplugged) before you assume a drive failure.