Well I have an old Seagate 1TB drive I've been using to store photos mainly on my photoediting workstation. Been using USB as this thing from Seagate has mini-USB, firewire and eSATA. The iMac has USB and Firewire, but the pins are different. The iMac looks like it has Firewire 800 and the little Seagate has a much smaller connection. It is either a 4 pin or a 6 pin. The USB is USB 2.0 so up to 480Mbps so about the same as Firewire 400.
Net, net, it used to be Firewire was way faster than USB, but with USB 2.0, you have to go to Firewire 800 or iSCSI or maybe eSATA (very hard!) to see a boost (about 2x)