I have a drobo S generation 2 attached to a mac running Sierra (10.12.3). The file system on the drobo, in case it matters, is HFS+. One drive has failed, and I cannot get my data off.
It has long been true that if I attach via USB I can attach to the dashboard but not mount the drobo, if I attach via my e-sata to thunderbolt bridge I can mount but not attach to the dashboard, and if I attach via both I can neither mount the drobo nor attach to the dashboard. My firmware version is 2.1.5 [5.41.62910], and my dashboard is version 3.0.0 
All my disks were 2TB drives, and I was using single disk protection. The top drive failed (so said the blinking red light and the dashboard), and I can no longer mount the drobo.
I replaced the failed drive (which seems to work fine in another enclosure, BTW) with another 2TB drive. When it is not rebuilding, the light next to the new drive is red.
I replaced the new 2TB drive with a new 4TB drive. The drobo recognizes the 4TB drive no problem – no change in behavior. The drobo displays a solid green power light, then all 5 drive lights turn yellow. The drive lights turn off, and I see a blinking green power light along with running blue lights. The blue lights turn off, then the status light turns solid green while the power light blinks, then the power light turns solid green and the drive lights come on – 4 solid greens and one solid red. Shortly after, the dashboard finds the drobo and tells me to replace the 4TB drive with a larger hard drive. I scoff at it – 4TB is my biggest drive. The blue lights slowly turn on from left to right, while the dashboard indicates that more and more data is being used.
During this phase, Disk Utility does not display the drive, and it does not mount (regardless of how I have it connected). “diskutil list” on the command line shows the drive and an 8.8 TB data partition, but “diskuil verifyVolume” claims the partition is corrupted (Underlying error: 8: Exec format error).
Shortly after that, the lights begin to flash green and yellow; the dashboard indicates that data protection is in progress. This lasts for approximately one minute, the disk lights flash 4 red with one white, then all give turn yellow, and the cycle begins again.
Sadly, this loop does not last forever. When I leave it for a few hours, I typically come back to an amber power light.
I have powered down, rebooted, and reconnected both drobo and computer. I have unplugged the drobo and let it sit for 15 minutes. I have tried removing all drives from the drobo and resetting (twice). When the drives are removed, Disk Utility can find the drobo. I have disassembled the drobo and air-cleaned all components. I have tried running the drobo without the computer powered on. I have tried placing the drobo in read-only mode. No improvements.
I have no further ideas. I quite want my data back – some of it is not backed up elsewhere. I don’t really want another drobo, but if I knew for sure that I could get the data back, it would be worth it to me to purchase another drobo enclosure and pull the data.
Does anyone have suggestions?