I have a 320gb and 500gb My Book and both work well. They are the older, no frills unit with the circle as shown above. To avoid problems I never leave them on or in the 622's usb ports. When I want to archive, I plug the unit to the 622's front usb and they turn on automatically. When I'm done I simply take remove the usb cable.
Right now I am archiving a couple HD movies to a new device, it is a Simpletech SP-U35, 500gb EHD. I bought it the other day on Amazon during a sale for $99 and free shipping (I think it is currrently $114 and free shipping which is still a good deal). It seems to be taking the transfers fine but we shall see if playback is problem free. A few others have tried this drive and have had no complaints. It comes in colors (I have black and it is sleek and cool looking). It even has a blue circle on the face that tells you how much storage is left.