I want to follow ScaryBob's recommendation and add a Western Digital WD10EADS to my 6131, and my local favorite geek shop, Memory Express carries it, but on their website they list it as a 5400 rpm hard drive. Link here: WD10EADS (http://www.memoryexpress.com/Products/PID-MX23334%28ME%29.aspx)
I called them, and the sales rep told me that the 7200 rpm model is the Black model (http://www.memoryexpress.com/Products/PID-MX23334%28ME%29.aspx) and the Green models are only 5400 rpm. Seems to me they are likely mistaken but I don't want to take a chance and get the wrong one. They have it on sale until tomorrow so I have to act fast.

The Green drive is indeed a 5400rpm drive, but the data transfer rate is more than adequate for your purposes. People are having good success and compatibility using those drives for an external PVR drive.

I am using the WD Caviar Black, which works fine as well.