Are you running Windows 8 and have a load of old unused hard drives hanging around doing nothing?
Use Windows Storage spaces to make a redundant RAID array that’s really easy to manage, best of all it doesn’t matter what varying sizes the drives physically are.
You can use any mixed sized drives you have to hand plus if any one of those drives fail your data is still safe while you replace it.
If you don’t have a secure backup solution currently in place this may be the answer for you.
I’m going to build myself a little standalone machine out of old spare parts i have just for this purpose instead of an expensive NAS enclosure and expensive matched capacity drives, then tuck it away somewhere out of sight and sound and access it via my gigabit network.
This setup is unlikley to be useful for use with Pro Tools as it does not support software RAID but would be great for a secondary backup destination for your projects, sound libraries, installers and other files you need to keep safe.
Coupled with some form of cloud backup service I think you will be pretty well covered.
See it in action in the very well presented video by TrainSignal/PluralSight
Thanks also to Jon Christopher for pointing out to me a similar Application is available for OSX users called ZFS see here for more details