You may have to upgrade to 8.6. Other than that, you should be able to use any USB pen drive. Depending on your iMac, you may be limited to USB 1 speed.
FWIW - I use a pen drive on my 10 year old 9500 all the time. All I had to do was add a USB PCI card. All iMacs have USB ports, so that won't be a problem for you.