Had one WD go flaky on me about 4-5 monthes ago. It kept losing it's functional status any time the computer was off more than 5 minutes. The raid 5 configuration would rebuild which took hours, first time I thought nothing of it then notice it would always go out after a power down. I went out and bought a new drive and hasn't happened since. RMA'ed the bad drive and now I got a hot-spare.
Had a maxtor also fail, this was after the first time I left that computer off over night. Went to power up and it went clunk, clunk, clunk. I'm afraid to turn any computer off now.
KK