External Hard Drive for old iMac

Hello! I have an ancient iMac (20-inch, Late 2006) that I'd like to connect an external hard drive to. Before I buy one, though, I need to know if the hard drive will work with the outdated OS. It's currently running Mac OS X Lion 10.7.5. I can't update it any newer than that, do to it's age.

Does anyone have any recommended external hard drives that will be compatible with the old operating system? It appears the hard drives sold by Apple require a much more updated OS to work.

Thanks!