What kind of hard drive is inside a Western Digital external product? (wdc.custhelp.com)

That's not what the linked article says. They won't commit themselves to a specific drive model in a given USB enclosure, that's the only reason for not specifying a speed.

It'd be better to have an actual measure of the unbuffered read rate after a seek. That's pretty much all the spin rate is good for, unless you're worried about its angular momentum or something.