hmmm… sounds like onsite visit is necessary. Try not to make any more drive changes (Except that one red slot) to facilitate protection of your existing data.
I know that once a hard drive has been “Seen by” the drobo, it marks it with a special tatoo of sorts, which could be complicating your testing and resolve.
For example, if you download the freeware (GPL) software called “DBAN” (Dariks boot and nuke) which is made to wipe hard drives of all data, you might be successful in removing this drobo “signature” on one or more of the disks that you have previously installed in there with null results … and very possibly that could help turn your red frown upside down. Just a hunch.
I did a test one time with my B800i’s, where I took 4 existing drives from a previous drobo (of an 8 drive array) with data that I didn’t care about, and put in 4 new drives, and booted the drobo. The fun of “ALL RED LIGHTS EVERYWHERE” was expected and confirmed. I of course didn’t care about my data and knew that the drobo realized that some of the drives had the signature, but “too many” were removed for any data protection to survive. Through drobodashboard, I factory reset my drobo and it cleared out the all-red, alebit, my data was knowingly “all gone now” and it was zero percent full. Again, this was just my test, with data I didn’t care about, so I’m NOT recommending this in your case.
While that’s quite different than your situation, it proves to me that if drobo thinks certain drives should be in there, if you swap out multiple drives, now they have some sort of drobo signature on there that may be further confusing the drobo. If this is the hypothesis, the DBAN trick on the single drive(s) that are not resolving the red light (just zeros, quickest way to blank a drive is necessary… although DBAN permits choices of wiping with DOD layer algorythms) allows for the drobo to hopefully try that “virgin drive” and you just might have better luck.
Or if the port is indeed damaged, well, that very well could prove it the other way.
I would caution AGAINST relocating any of the “known-good, working” drives as they have your data on it.
And finally, to add to what rdo said above, the “cold boot” is what I also recommend, the power button off, and then couldn’t hurt to also unplug A/C power for a few minutes, just to ensure that it restarts with a clean slate and see if then it can improve it’s ability to utilize slot1.
You didn’t previously answer, do you have drobocare on the unit?