Re-use replaced backup harddisks 
Now you have RAID system. It runs perfectly, but it also runs full as all storages do over time. You buy new 1.5TB harddisks, replacing the old 500GB ones. Now what do you do with those old ones? They are still perfectly healthy disks.
Well, you buy an
external SATA dock!
Then you can do off-RAID backup to the disks. Those disks probably last longer than your DVD backups.