This originally was from the old blog. I’ve reposted it with a few minor changes by request.
Make your own Drobo for cheap. Well, not exactly. But close. If you are like me your main disk drive is 3T or bigger now. You also probably have a bunch of 1T drives laying around. Some you can use for a particular manual backup of data. However you can’t really use them for automated backup because they are just all too small. Like me you may have looked at Drobos or Synology bays with lust, wishing you could take your old drives and make a backup system with them. Yet those drive bays are usually ridiculously expensive. Too much for Time Machine. 1
The alternative is to use Apple’s Fusion Drive technology. You won’t be creating an SSD/HD fusion.2 Rather you’ll be taken a bunch of drives and treating them as a single logical drive. To avoid things from getting too confused, I strongly suggest getting a multidisk bay. I used a ProBox USB3 4 bay enclosure for mine. If you don’t do that remember that if one drive goes down none of the drives will work right.3 In other words bad things happen if they aren’t all running. Trying to keep 4 disks straight when you may have several others is just asking for trouble.