After letting WU run its course and enabling write caching on the disc, it's now running nice and fast like it should.
I'd still like some advice on checking that I have the right drivers, though. The device manager just says my HDD is an ATA device. It doesn't say whether it's SATA, SATA2, or PATA, or whatever, and from what I gather these discs can run in any of those modes...? So how do I know which one it's in?