I know this is late, but you are wrong. While a differential backup will need to be taken multiple times, it should not take that many drives to record data considering this backup consists of one full backup tape and a differential tape for the files that change. You would be able to use the same tape to backup each time since only a small change is made, meaning you would only need 2 tapes. A full backup isn't necessarily 2 tapes, which is why the definition includes that you should follow a 3-2-1 rule, which is 3 backups, 2 on site and one offsite. Always check your information before posting online, because you can mislead and confuse people.