From the knoledgebase:
Running the Repair:
NOTE: Since the network attached Drobo’s use a Linux-type repair (e2FSCK) (File System Repair), which can be destructive, potentially damaging data. We recommend you block level clone each drive prior to beginning the repair if your data is critical and you have no backup. For more information on Cloning, refer to this knowledge base article.
Once you are ready to run the Repair, go to:
Drobo Dashboard > Tools > Repair
You will be prompted to PROCEED. At this point the Drobo will run its internal file system check (e2FSCK) and you will see the message “Repair started successfully”.
Depending on the degree of damage to the file system, the repair may run for several days, weeks or months. If you have a backup of your data you can choose to reformat you Drobo, understand this will erase any existing data on your Drobo.
Once the repair is complete, you will be prompted to reboot your Drobo. If you are not prompted to reboot your Drobo, go to Drobo Dashboard > Tools > Shutdown, once Drobo is completely powered off you can now turn it back on. (there is no progress bar or indication with respect to how long the repair will run).
If the repair continues to fail, you can try running the repair once more.
If the repair still continues to fail, do not reboot your Drobo. Generate diag as follows: Drobo Dashboard > Help and Support > Get diags