Time Machine and external Hard drive...space usage?

You don't have to partition at all. Time Machine makes its own folder, completely separate from anything else on the drive.

 

Apple describes how Time Machine works in the Time Machine preference pane. It says that that eventually Time Machine stops saving weekly backups when the disk is full. You might want to watch out for that. Time Machine will take up all of the free space that you leave it.

 

But it will not overwrite other files on the drive.

 

No, but the OP wants to allocate half the drive to video. In order to keep half the drive free, he will have to partition it. Also, this other partition will not be backed up.

 

Good point.

 

I just created a blank disk image of the space that i didnt want time machine to use