The hard disk market is very competitive, and it's not really our intention to sell any HDDs alone. There are also risks in shipping these alone: damage; loss etc as they are quite fragile. There is also no way that we could compete on price, even with concessions from WD.
Re. selling the enclosure on its own. I agree that many would be happy to put the disk in themselves, but the issue is the type of drive that the enclosure accepts. The enclosure accepts a USB hard disk instead of a SATA one, which I think may catch a few people out.
Perhaps we should make the enclosure more unique, and make adding an HDD an option at the checkout.