I want to get all the external hard disks off the Mac Mini, so I don't need a USB hub. The Drobo is full. I ordered two new drives from Newegg tonight. I ordered Seagate 4 terabyte drives.
I've been following BackBlaze for a while, reading their reviews of hard disks. The Seagate 3 T drives had much higher failure rates, and I've been avoiding Seagate for a while. But their 4 T drives are getting very good reviews. So I ordered two of them. Cost $280 with free shipping. That should give me a total of 9 terabytes of storage in the Drobo, which will let me eliminate those last two USB drives.
They will arrive next week and then we will see what happens.