I have roughly 300 GB of current data that I backup fully once a week (to LTO Tape), and do incremental backups on the other days. Additionally, I have about 300 GB of inactive archive data that is still used occasionally. It is backed up once a month to LTO tapes. The monthly tapes are kept off-site indefinitely.
Because our server drive space is limited, inactive data that has been backed up 3 times is erased or transferred to external USB drives located in our departments. Data that has been transferred to the external drives is, naturally, no longer available to external employees using VPN.
We are thinking of using a 2TB SAN or NAS to put both current and archive data on, never erasing anything.
My problem with this is the backup time needed to backup 2TB of data. We use an LTO2 tape drive that could theoretically backup 2.4 TB.
It would probably run out of time to backup the data (only over the weekend).
I have read about the other possible solutions (mirror servers, for example), but then lose my ability to re-create data when catastrophic failure occurs (fire, earthquake, etc.). Since both servers would be located here, the fire (for instance) would most likely ruin both systems.