The Western Digital site says that it "Utilizes a WD drive with power-saving WD GreenPower Technology". That would mean that it's almost certainly one of these. You might want to check if you can get the bare drive (if it's suitable) cheaper, but it seems to be out of stock at a lot of places.
I've got a 1TB version of that external drive. The unit is sealed (no screws or such like visible or under pads). You'd probably need a sharp stanley knife or junior hacksaw to gain entry. Other external drives I've had could be opened relatively easily.
It's probably not the WD15EARX since that's the SATA III drive and there's no point putting that in an enclosure with a USB 2.0 interface when SATA II is fine.
Also I know that the 2TB external drives have WD20EADS drives in them (i.e not quite the current Caviar Green) so it's quite likely to be the WD15EADS
That said I don't think you can ever be 100% certain what drive you'll find in this sort of thing. I'm sure I read one review where they found another manufacturer's HDD inside. Since they don't make any actual promise as to the enclosed drive it might well be what they have available/what's cheap for any given production run.