For the record, my 1TB MyBook drive is working fine. I was asking in case I want to use is as an internal drive, or in an eSATA enclosure. Then I could give the USB enclosure to a friend.
I have lost a lot of WD drives, and I am just an end-user. Two WD5000KS drives bit the dust almost simultaneously, with no physical abuse whatsoever. I RMA'd a third just-in-case. One WD5000AAJS was DOA. In recent years, I have used and resold four 160GB and two 250GB drives which had no problems. Still, that's a high failure rate, and it cost me a lot of trouble for backup and data recovery. What does 1 million hours MTBF mean any more?
It's easy to blame temperature, especially after touching an operating drive. Given tight tolerances and dissimilar materials, I also blame temperature fluctuation. It's very difficult to arrange for air flow over drives in most PC cases, and these drives get HOT, even doing little. I'm glad to see the new GP models with automatic turn-down, and I hope for longer life along with power savings.
Interesting news from Google research:
http://news.bbc.co.uk/2/hi/technology/6376021.stm