I have had good success with the WD MyBook drives until recently. I took my HVX200a, my laptop and an external drive as my carry-on items for a recent shoot in Colorado. I shot for a day, and went to off-load my P2 cards to the WD drive. When I plugged in the drive, which contained lots of P2 backups, it kept wanting to format the drive. Since I needed the footage that it contained, I was in a bit of a pickle. Luckily, I had enough space on my laptop to dump off to. I went to the closest Wal-Mart and bought another drive. When I got back home, the drive seemed fine. About a month later it did the same thing. BEWARE!
JPD