Your drive is full because you have to many saved recordings.
You only need to delete the ones you no longer want to keep.
If you think about reformating, whitch is not needed, then, most if not all recorded programms are old things not worth keeping.
Now, take some time to go through your schedule and remove some recurent recordings. Maybe you forgot to remove some schedules for shows that you don't realy, or no longer, want.
Also, after you watched some recording, you should have the option of deleting it. If you don't intend to watch it again later, DELETE watched recordings right after you finish watching them.