Time Machine How to change back up drive temporarily

Hi,

I have an additional HDD in my powermac and TM is set to auto back to this but what i want to do is run a backup to an external HDD from which i will import the data to my parents computer so that i have another backup of my stuff in a different location for safety's sake but i didn;t think you could change the location of the back up on a temporary basis and that all you could do is run a fresh backup on the external drive and re-run another back up which will take ages again onto my existing hdd.

any help appreciated

thanks