Currently Being ModeratedSep 8, 2012 12:00 PM (in response to Gpcn)
Advice can depend on whether you intend to use the drive on a portable computer or a desktop. It also can depend on the Mac model. This forum covers Macs made before 1998, and thus few of those Macs have built-in USB or FireWire unless it's a minitower with available PCI slots.. For those you can get a PCI USB expansion card, a FireWire card, or a single combo card that gives both.
So it will help us help you of you can post you madle model, processor type, and Mac OS version.
Most portable drives can only get power from the computer's USB ports. Some ports fail to provide enough power to run the motor in some drives. If you need this for portable computing when you cannot get extra power from the mains, you may need to invest in OWC's USB "Y" cable:
It attaches to TWO of the computer's USB port to get more power to the drive. Usually fixes the common low-power issues with portable drives. It's a cheap investment.
If for desktop use only, avoid the portable drives and go for one with a standard 3.5-inch drive and an independent power supply. People seldom have connectivity grief with these, and the 3.5-inch drive ares more durable than the 2.5-inch versions istalled in most portable drives.
For older Macs with built-in FireWire or FireWire ports on a PCI card, always get a FireWire drive. FW 400 is functionally faster than USB 2.0 and pre-Intel Macs can be booted from a FireWire drive in an emergency. Again, the portable drives seldom have FW ports.
The Mercury Elite Mini is a hybrid in that it has FW400, FW800, and USB 2.0. If also has a separate power supply as an option: AC power adapter (not included). The only downside is that it's the small-format 2.5-inch drive. Other than that, it looks like a good choice
Intel Macs can boot from a USB 2.0 drive.