Newegg has Western Digital My Book Studio 500GB 3.5″ USB 2.0 / IEEE 1394a / IEEE 1394b / eSATA External Hard Drive Model WDH1Q5000N - $89.99 + free shipping.
3.5″ Form Factor, Capacity: 500GB.
Interface: USB 2.0 / IEEE 1394a / IEEE 1394b / eSATA.
Automatic backup, SmartPower features.