Would Firewire 800 be faster than USB? Could you please tell me the specifications of what I have to buy? Do I need an additional external power source for the HD?
harryb2448 - confused... formatted Mac OS Extended - I have no idea. How can I tell? What does that mean?
Not wishing to butt in on Harry's post, but 'Extended' means formatted so that you can boot (start) your iMac from it if it has a MacOS.
Because you don't want to swap the drives, then a firewire enclosure (e/e) is the way to go. There are several brands, but the best place to start looking is at OWC
Performance Upgrades; FireWire USB SATA Storage; Memory, more at OWC (I'm on very slow dialup at present, otherwise I'd provide a more direct link, but check External Storage).
One thing you should look for is firewire (or firewire/usb combo) e/e's with an Oxford chipset. OWC nearly always tells you if an e/e has Oxford. If it doesn't say so, it's unlikely to be so. Oxford is very reliable, and works well with the Mac hardware. Look carefully at diagrams of enclosures to see that they have a FW800 port. Some have both FW 800 and 400 ports, but 800 is backwards compatible with 400 anyway.
Yes, you should use an externally powered enclosure, USB power from your iMac is inadequate.
As MacDude says, Firewire 800 is fast - you'll be satisfied with its performance. Forget USB, except for plain data transfer.