hi prregor, sorry to hear of your problem,
it sounds as though 1 or more drives encountered some problems during the expansion process and that could account for the reboot loop (or rebuild loop)
if dashboard can still see the drobo, and if it does show you an estimated time remaining which is getting lower, it might be worth letting it take place a bit more, though how much time is it showing remaing?
it shouldnt just keep rebooting the unit over and over, though can i also check about those hard drives… for example when you mention they were checked in linux, and that they were ok, was this in the past, (or have you actually powered down and taken the drives out and retested them since having this issue?)
while it is looking like a rebuild, i would be inclined to let it run through the rebuild process with flashing yellow and green lights (or orange/amber/yellow and green) at least for as long as 1 pass of rebuild would take… which is usually about 1day per 1TB of data that you have on it plus a bit)… though if it is the reboot loop issue, then one thing you could try later would be to try putting the drobo into a read only mode, which could help to stabalise things to hopefully be able to at least access your data to copy off somewhere, for example as mentioned here: