There are external hard disk cases with different connectors available: PCMCIA, USB and FireWire. Cases are available for 2.5" (laptop hard disks), 3.5" (desktop hard disks) and 5.25" (CD-Writer). All of them work very well together with Linux. Especially I like the cases for 2.5" hard disks, you may upgrade your current laptop hard disk and use the old one to put it into the external box to extend your hard disk space.
Caveat: After wake up from suspend mode, the external hard drive can't work. To cure this problem these remedies might help: Disconnect the external drive and then plug it in again. Or use an AC adapter to power the external drive. Though this seems unconvenient in a suspend situation. But since the external drive gets the power from the adapter, there is no disconnection from power as will be if power is provided from USB.
Caveat: Take care that the jumpers are set to Master. Almost all external hard disk cases will not work when the jumpers are set to Slave or Cable Select.