Hi
Before purchasing additional Disk advanced licensed for the file library, you may want to look at where you want to be in 6 months/1 year time:
6.20 is end of support life soon.
Synthetic backups have been great in version 6 up to 6.21 but it is all broken since 8.0 (dont know about 7.x) up to the latest 8.12.
Now consolidations fail completly after 10 to 12 days, requiring to manually perform a full backup to get you going for another 10/12 days.
If you have a minor error during incremental, there is a huge chance that the whole server will be missing on the next consolidated backup. There is no error to let you know it happened. You just find the hard way if you need to restore, or if you look closely at the logs every days.