A DLT tape drive should do the trick, but they are pricey.
However, some more questions. It's unusual to have that much data that needs to be backed up and only be using NT backup. You may want to review you backup strategy
Is the 70 Gb all on one server ?
Is 70Gb the total capacity of all the drives on the server ?
Is 70Gb the amount of data need to perform a FULL back up ?
What backup style do you use ? (Incremental / Full) ?
Pete H