I have tried the following 250 GB hard-drive and it works without any problems so far, going to fill it up and see what happens. It is cheaper than getting a 128GB or 256GB USB Flash drive
http://www.newegg.com/Product/Produc...82E16822136417
They are out of the 250GB now only have the 320GB, this should also work , might take a little extra time during startup than the 250GB. The 500GB in this series also works but takes a long time to start playing the music. 250GB takes about 7-8 seconds.