quick hd question

Hi, I have just bought a new mac mini and set up a wd passport as time machine, it backs up around 30gbs.. i have deselcted music and video folders.

so there is 290gb free, can i use say, 200gb of this space to store my music and videos permanently on the free space without the need to constantly back up

cheers

Yes, but you'll have to partition that external with one as Mac OS Extended (Journaled) for your T.M. backup (as you know) and use the other for media storage.
If you want to manually drag-drop your media to the "storage" partition and won't be using Time Machine to backup to it, it can be formatted as the same as your T.M. partition or as NTFS (if only your Mac will be copying to it).