The first challenge will be getting the enclosure open without damaging it, as there are no screws, just hidden clips holding the two halves together.
If you do manage to get it open without damage, you may find that the existing drive inside it has a proprietary connection interface to which the SATA/USB bridge-chip is attached. The drive itself may not have a standard SATA interface.
You will just have to split open the enclosure and find out if it's possible to swap the drive.
They have standard 3.5" drive sata connectors, any 3.5 sata drive will work.
If you took the original drive out and want to replace it with a different drive, you have to install the WES driver and then download and run the Windows quick formatter program from Western Digital.
There might be another way such as a few of the linux boot cd partition tools like gparted, but this is the easiest way.
I found the My book 2tb drives on close out at Staples for $41, bought 4 of them just to steal the drives for other purposes and I'm now using the Western Digital cases for extra storage with s few old 500 and 300 Gb drives I had laying around.