You can use any HD including Western Digital if you modify your Network Adapter, apparently.
I think Seagate drives fit without modification, and I'd probably use on eof theirs anyway, if I didn't have the official Sony one. Honestly, the Sony one should be sufficient unless you're planning on pirating a bunch of games, which I don't recommend! The benefit of the Sony one is that you can create a backup save folder on it in the memory card menu and copy all your saves for storage. I'm pretty sure you can't do that with non-Sony drives. I got mine used at EB for like $45 after discounts.