The two 4T hard disks arrived today. The Drobo has been waiting patiently, telling me that the first 500 gig drive needs to be replaced.
I removed that top 500 gig drive. The Drobo started to flash lights. The remaining drive lights slowly flashed green and yellow. I was warned to NOT remove any other disks or data loss would occur. I inserted the first of the Seagate 4T drives. It was a little stiff to get it inserted, but it went in, and it clicked into place. The lights continued to flash green and yellow. I was told that I could access data, but I should still NOT remove any more drives. A progress bar started, and it told me that it was rearranging data and it would be done in two hours. I got on with other things. And sure enough, two hours later it was done. I had 4T in top bay, 500 gig in second bay, and 3T in third and fourth bays. That gave me a total storage of 5.85 terabytes.
I let it settle for a half hour, then I wanted to replace the last 500 gig drive. I wasn't sure about just removing a disk when it wasn't telling me to remove a disk, so I watched some Drobo tutorials on YouTube and was reassured that I could do it and not ruin everything. I removed the last 500 gig drive, and inserted the 4T drive. That was hard. It did not want to go. I had to take it very carefully and get it aligned just right and then it slid into place. Once it clicked in, the green and yellow lights flashed again. Rearranging data would take two hours. I let it run.
Two hours later it was all done. No more flashing lightsk, and 9 terabytes of usable storage.
So now the Drobo is well situated. I have a total of 9 T space. I can now get on and store the contents of the remaining two USB external hard disks on the Drobo, and then I can remove the USB drives and the USB hub. That will leave me with 7.5 T data on the Drobo, and 1.5 T free. Then I can decide what else I want to back up from my Linux desktop. If I need more space, I can replace the two 3 T drives with 4 T drives, and that will give me one more terabyte of free space.
In time, 5 T and 6 T drives will become available at reasonable prices, and Drobo will update the firmware to accommodate them. By that time, I might be better off getting a five bay Drobo.