i think that while there might not be official support (from a drobocare package point of view) if that model is quite old, i think there is still a pay-per-incident support ticket that could be raised with them if you would wish.
i dont actually have a drobo pro but from what you said, if the #2 drive was flashing (usually indicating a failed drive) or was just solid red (usually indicating low overall space in the drobo), then both could simply be good reasons to replace it, like you did…
in your case though, the usual process to do this is to usually remove a drive (while power is on) and for drobo and dashboard to be able to recognise this fact, and to then put in the replacement, (also while on) for drobo to shortly after recognise this as well.
what may have happened is that by doing this with power off, it may have confused the unit.
(does it still become recognised by dashboard, and the computer currently? am guessing not but just to check)
in case not, it is good that your drives are backed up, and especially with this in mind, you could probably try a few things…
one option, if you can, it might be good to try and wait for about 1day per 1TB of data that you have on your drobo… just in case it actually is trying to reverify the data but not actually showing a usual rebuild of yellow and green lights)
then, if you can not wait much more, you could try to power it off, and swap the 2tb back with the 1tb that you removed when off, and then power it back up again. (this may allow it to wake up with its last known configuration again)
another option, could be to try powering it off, removing the drive #2 , and powering it back up,
this should in theory be the same as though that drive failed, with the other drives still containing your data. (unless some rebuild had actually taken place)
(in the worst event, it may require a full reset and copying back your backups to it, though there might be some more steps that can be tried before then too)