I had four Drobo four drive network drives. I started out with 1TB drives. The entire thing was up to 16 bays of 4 TB drives.
I was backing up stuff at least twice.
What I did not do was a good job of indexing individual files. Finding an event is a task and a half now.
I upgraded from Seagate to WD drives because the SGs were burning up. I never lost data, but I hated tossing those drives away.
To solve your indexing problem, look for a free program named Cathy. It will build an index of all of your drives that is searchable. Since the index is on your hard drive, you can index a removable drive, remove it and still have it included in the index. I have about 20 or so off line drives that I have in my index.