So now I'm in the market for large internal hard disks to put in the Drobo, Doug supplied this link where Backblaze analyse their hard disk usage. I read their 2013 report, and this is the 2014 report.
They aren't thrilled with the 3T drives. Seagate had a really bad run with 3T drives. Western Digital were okay, but still too many failures.
With the 4T drives, the situation changes and the Seagate 4T drives did really well. I'll keep all this info in mind. But right now, 3T drives are most cost effective, so that's what I'll be looking for.
And today Newegg sent me one of their regular promos and these Western Digital Red NAS 3T drives were on sale at $109 with free shipping. I bought two of them. I'll wait a couple of weeks and get another two. I don't want to get four all from the same batch. There's a chance they will all start failing at the same time.
I'll have that Drobo fully populated soon.