thanks for more info,
ah the test i did was just to power up my 5D and try copying some large files to it to see what explorer showed me. (not the most scientific but just wanted to see what it would show me as a quick comparison bearing in mind that i had less free space so i fired it up to do a quick test)
as your drobo is now showing a mismatch of lights/status (compared with the dashboard) then there could be something wrong like you suspected… if it is the bay (slot) though, then it is possible that the backplane has suffered some damage (sometimes possibly if the unit was moved with drives inside or tilted), but if dashboard still recognised the drive, (and shows a green light there) as well as the correct into for that drive model and a good status, then it could be a faulty light…
(for the drive model info, newer versions of dashboard also include another dropdown / tab which shows you drive info for each drive bay that you click on)
are you able to take some screenshots of dashboard (with bays) and to post up to imgur or similar, with links pasted here if you can? (please remember to rub out any sensitive/serial numbers before uploading though to play safe)
can you also remember if your drobo is using single SDR or Dual drive redundancy mode? and also, when you are not doing anything to access drobo files, do the drives in the drobo seem to make lots of chatter noise/vibrations (similar to like when a hard drive is reading and writing?)
if all is ok with the drives (and dashboard), and if it is only the main drobo unit LED light that has blown, then you might be able to carry out a migration process to essentially swap out the diskpack into another replacement model, for example as mentioned here:
http://www.drobo.com/resource-center/migration/
can you remember if the drobo speed increased a lot, when you had removed the failed drive? if so, maybe an option could be to remove the drive from the seemingly-faulty bay, and to then copy your needed data from drobo to somewhere else such as your mac, much more quickly. please note: if you have any really important/critical data on it at the moment, it might be worth trying to make a backup copy of it (even if slow) before trying a drive removal.
you might be out of support (and seeing the pay-per-incident) option, though if you were to buy a new replacement, you might be able to get some support for that (which might be able to help confirm what the current problem is, but in case not, it might still be worth checking and confirming the status of your drobo with support, possibly via diagnostics logs at certain stages that they might ask for, just in case there is something else causing problems that might still persist after a migration process, such as possibly problems with some other drives too?)