Clear data from HD

I have had to reinstall the drivers a few times and each time that I do I then run backup again.  And each time the old data on the HD stays and the new data is added.  I am running out of space on the external HD even though my computer only has about 400GB of data that needs backing up.

How do I clear the ext. HD so I can just have my current data backed up?

The drive should not ask for drivers, every time you connected to you computer, have you scanned your drive for errors?