It depends on the model. You can search for "FreeAgent ST305004FPC1E2 cache" on google. Replace ST305004FPC1E2 with the manufacturer's part #.
16MB cache model:
Seagate FreeAgent 500GB Esata/USB 2.0 16MB CACHE 500 GB Hard Drive - Find, Compare, and Buy at DealTime.co.uk
8MB cache...