My friend bought an iMac that has only Thunderbolt and USB2 connections for an external backup up drive. I'm supposed to

[Black]

GeneL wrote:
I'll have to wait until I can get over to look at her computer again but after all these years, I should know if there was a FW port on the back of her iMac.

I didn't see one, but I will check again, if I can wake up during the day.

Don't bother. FW is a needless added expense and limitation when looking for a current backup drive.
Get any USB drive that's on sale, preferably one with a 'hard' power button.