Is your Drobo up to date? Don't trust Drobo Dashboard for the answer.
Drobo and I have not been getting along this week. I have two Drobo units: a FireWire model and a USB model. I moved them to my office last week, and the USB model started complaining about being low on space. The Drobo Dashboard showed drives in all four bays, and since I hadn't been low on space before, I was confused as to what had happened. About a day later, I checked again (after working hard to delete anything unnecessary) and it showed a missing drive in one of the bays. Not a failed drive, but a missing drive. I shut the Drobo down completely, ejected the drive, and mounted it back on the iMac.
Suddenly I had a lot of free space, but Drobo was acting like I had just put in a new hard drive. It took two days for it to sort itself out.
Everything seemed fine until I was using my iMac a few days later. Suddenly, my iMac said "You have ejected [my FireWire Drobo] without unmounting it." I told it that I had done no such thing, I hadn't ejected anything. I couldn't get the Drobo to remount, even after rebooting. It would work via USB, and it would work via FireWire with my MacBook Pro, but not with the iMac.
I worked with AppleCare to diagnose the problem, and they asked me if my Drobo firmware was up to date. I said it was, and that it was one of the first things I had checked, using the latest version of Drobo Dashboard. My answer was true... except that Drobo Dashboard wasn't telling me about the latest firmware.
I spent all day on Friday talking with AppleCare, running diagnostics, driving back to my house to get my original install CDs, calling AppleCare again, running more tests, and finally having them agree to send a technician out to replace the logic board on my iMac. He arrived this morning (Tuesday) and spent an hour doing the replacement.
About an hour later, the Drobo started acting up again. The install of Snow Leopard was only a week old and I had a completely new logic board, so I was starting to suspect the Drobo. I called Drobo support and their tech asked me if I was running the latest version of the firmware.
"Oh yes," I said with confidence, "I checked in Drobo Dashboard this morning."
"There's a newer version on our website," he replied.