There have been several cases reported here where the Drobo came up with 4 red lights (too many drives removed) after upgrading a drive with the power off. I believe the last case reported here ended with an array loss, despite a two week heroic battle to save it :-(. In all cases the new upgrade drive was previously part of a Drobo pack although it may have been reformatted in the interim.
We speculated that the Drobo happened to pick the new drive as the “pack reference” and then determined that the other 3 drives were from some other Drobo pack and not valid.
I agree that pulling a spinning drive is not a good idea. I don’t think drives were mechanically designed to be hot swapped like that and I don’t do that with my eSata docks, for example (actually they supposedly don’t like that but I wouldn’t anyway).
If you are concerned with this, then a better procedure would be to power the Drobo down, pull the drive, then power it up with a drive missing. Then when it is fully booted up, install the new drive. Now the Drobo is up and running, it knows the drive is a replacement and should deal with it properly even if it’s been in a Drobo in some distant past lifetime.
An alternative solution is to dress up in cammie, sneak up on it, and pull the drive when the Drobo is sleeping and the drives spun down :-).