The older Drobo models support single disk redundancy, which means when more than one drive fails data is inaccessible.
An error message ‘too many drives removed’ indicates a multiple drive failure. Firstly, though it’s worth reseating all the drives securely and checking if the same message appears and if all bays are red. If you can do a block level ‘clone’ of the failed drive(s), you may be able to gain access to your data.
With three drives or more, Drobo has a selectable option to protect against two drive failures simultaneously. This is known as BeyondRAID and is controlled in the settings. Drogo will automatically rebuild the array to the working drives with no user interaction required.
It’s likely that the drives in your Drobo were all added at the same time. So whilst it seems unlikely that two drive could fail simultaneously, it’s a lot more common then you might think.
To enable ‘Dual Disk Redundancy’ do to the Drobo dashboard > settings > dual disk redundancy. Once enabled the LED lights will flash green and amber. In older versions you may see the message ‘data cannot be protected’ will in data protection mode. Newer versions will show ‘data protection is in progress’. This can take quite a bit of time and it’s important not to power down until the process is completed.