Summary: Saved me! I had an internal hard drive failing and got this to use as a start up disk. Reformatted EASILY for a mac and reinstalled all new startup software etc. Also, (thankfully on a MAC), migration of all data went smooth. Took quite awhile since this is a USB drive, (not like if it were a firewire), but it all worked and works just fine. Certainly a lot less expensive than having to take it to the shop right away.
Summary: Had to call WD to use on my MAC because Macs don't recognize. They were very helpful and with a couple of clicks it was done! If using with a Mac and time machine, it copies all the applications as well taking up a lot of memory, but not nearly as much as is available. I had about 5 years of photos; a least a thousand or more and it took up 91GB of the 500. This should last a long time.