Need an External Hard Drive for my iMac Early 2008

Do you need the drive for backups or something else, like booting the computer? That can affect recommendations. What I've written assumes you need the drive for backups.

Your computer should have three USB2 ports. I recommend getting a USB3 external drive that is backwards compatible with USB2 for the 2008 Mac but will be speedy when you upgrade to a newer computer.

Having struggled with most of the "name-brand" external drives that go on sale at the office superstores every weekend, I have concluded they were designed for go on sale every weekend, not give long, reliable function.

For a desktop computer, I recommend a full-sized (3.5-inch) drive enclosure with a quality rotational HD inside and an independent power supply. That will be most reliable.

IMHO, a solid-state drive (SSD) is overkill for most backups like Time Machine, even on a new faster machine.

I use this model external:

https://eshop.macsales.com/shop/owc-mercury-elite-pro

You can buy one with a drive already installed, or a bare enclosure and install your own drive. I get the bare enclosures and install the WD Black series HD (SATA 6GB; 7200rpm) in them for our three critical-use Macs.

The specs for that drive say it is compatible with older Mac OS versions including OS 10.8:

If you are choosing an OWC solution, then I would recommend this OWC Mercury Elite Pro drive. This version has USB 3.0 (for a new Mac in the future) and FireWire 800, and on your older Mac, you can use FireWire 800 for faster connectivity than the other port on your Mac, USB 2.0.

Jack

I am a +1 with Allan. I think the OWC solution he mentioned is an excellent solution for such an old computer.