It is likely one or more of your drives are failing or had failed.
Perform a slot flush, individually, on all of your 5 drives.
The expected behaviour is the unit would still boot up, but the Drobo Dashboard would prompt “Too many hard drives had been removed”.
If at any point, if any drive causes the unit not to boot up, do that note of the serial no., you would be needing this for later. http://support.drobo.com/app/answers/detail/a_id/665/
If the slot flush is done successfully, meaning all drives causes the unit to boot up okay, shutdown the unit (Drobo Dashboard > Tools > Shutdown) and reseat ALL drives.
Boot up the unit, check whether it goes into rebuilding mode (flashing orange and green) on all drives.
Hopefully after it reboots, the data could be accessed, then immediately backup your data.
If the slot flush fails on one of the drive (meaning it causes the unit not to boot up), consider performing a block level clone of said drive. It is recommended to purchase drive(s) of the same model if you can, or at least similar model but larger in size (TB), if that model for it had ceased and is no longer sold. The important thing is to verify the sector size of that drive, it must be the same with the one you intend to clone the drive to. http://support.drobo.com/app/answers/detail/a_id/675/
Once the cloning is done, replace the drives with the one you had cloned and insert it back into the unit.
Hopefully this triggers a rebuilding (flashing orange and green), and once completed, the lights on the drive bays would show all green, and the data should be accessible once again.
If you can try to keep your usage below 80%, as any higher would have an impact on performance and accessibility too, if the usage is at its maximum.
Note: Do Not insert/remove any drives when the unit is turned on.
Thanks. I performed the “slot flush” as described.
Drobo Dashboard connected after every slot swap/restart. When I finally reseated all drives, the unit returned to the error state that I reported in my first post. It finds five slots and five 2TB hard drives, and yet “too many hard drives have been removed”