RPi does not have S-ATA, you have to use the USB connection. So you can:
1. Just buy a 2,5" USB hard disk
2. Go the complicated way and buy a 3.5" S#ATA disk, an USB to S-ATA adapter, a power supply and put everything together in a case (e.g.
http://www.ebay.com/itm/USB-2-0-to-IDE-SATA-S-ATA-2-5-3-5-HD-HDD-Adapter-Cable-/171391095725)
