I have an SP-808. I'm stupid! Where on the back does the external zip drive go?

Ok I've been Reading and reading all the posts. So I decided to change my 100mb internal drive Zip dr. to 250mb zip dr. I can do that no problem..But where on the back do you plug the external zip for storage/back-up? I can get a 250mb external USB zip dr. or a 750mb zip dr. and Plug it to my computer use it for storage and change my SP-808 to an Sp-808ex. Will a 750 mb zip dr write to a 250mb zip disc?