I have a Western Digital My Passport here from a friend. It's been dropped, and it's making clicking noises (uh-oh). I'm trying to see if it's recoverable, so I thought I'd remove the disk and plug it directly onto the motherboard.
After I read a couple of success stories I thought it would be simple. At least I'd have a free SATA to USB converter if all else failed. I removed the case and to my surprise WD is now manufacturing the drives with the USB port directly on the (non-removable) hard disk board.
Don't try and tell me this is necessary, the only reason I can see is to stop people (such as myself) re-using the drive in a computer, or using the enclosure with an upgrade / replacement.
I can't speak for your specific My Passport, but here are the details of this one for the Googlers:
The serial number is the same as the internal drive. This drive is stamped with the date 03 Dec 2009.
If you haven't bought a WD yet, don't expect to be able to replace the internal drive with a generic one!