If I transport my IMAC to be serviced, what should I do to prepare Seagate external backup drive?

Jack is correct, use Time Machine to back up, test the back to ensure it works and then ship the computer.

Welcome to the Apple Support Communities.

Make a backup of your Mac, then keep the Seagate backup drive.

Jack

If my Seagate has been used to back up my PC can it still be used to back up my MacBook Pro?

Your Windows drive is the wrong format for Time Machine.