Overwrite USB Storage Unit?

I am moving to USB 3.0 drives and away from tape.  I am curious if there is anything that needs to be done to ensure that the previous jobs on the USB drives are overwritten so that the job doesnt run out of space.

I imagine that the jobs should work the same way in that once the job is past the policy retention period the backups are overwritten.

Can someone verify this?

Anyone using USB storage units?