Pick your poison:
- Tape: inexpensive and slow, require frequent testing (backup we do, it's restoration the problem!), usually unreadable after 6 to 12 months or less (that's in production people).
- WORM: more expensive than tape and just as slow, work well in the medium term (meaning 10 years top).
- XFS NAS: faster than the above, require good hardware and a bit more work than either tape or worm. Don't forget to setup replication to multiple systems. May suffer from bitrot in the long term (checksumming/hashing files might be a good idea). Very stable, large capacity file system. Tape backup is always a good idea.
- ZFS NAS: slightly slower than XFS (at least, that's my experience, YMMV). Ultra-large capacity. Snapshotting is just a breeze. Again, replication to multiple, distant systems is mandatory. Very stable file system. Tape backup is always a good idea.
- DNA, 3D crystal lattice, holographic memory: what we are all going to use in the future. Still in beta testing, though.
- DVD: don't make me laugh.