I’ve got a Drobo S here with 5 * 2TB HDDs in it. There’s 1.49TB of free space on it just now and its connected over FW800 to an iMac running Yosemite. The disk pack is healthy, disk utility finds no problems and its been utterly reliable up until now.
In the last few weeks it starting behaving strange. I’ll come back to the iMac and there will be warnings about the volume having been disconnected without ejecting it first. The Drobo itself will just have the orange power LED on it so it looks like its switching itself off.
To get it back on, I either have to disconnect the power cable or use the button on the back to turn it off fully and then back on.