I moved a 1TB drive off to another controller. I installed a 2TB (already cleared) into where the 1TB drive was. Upon booting it the old 1TB drive is unassigned and the new 2TB is automatically "upgrading disk". Do I need to wait until the upgrade is complete before I can assign the old 1TB drive to the array? When I tried to do so, it gave me an "invalid expansion error". So I unassigned it and left the new 2TB "upgrading".

I moved a 1TB drive off to another controller. I installed a 2TB (already cleared) into where the 1TB drive was. Upon booting it the old 1TB drive is unassigned and the new 2TB is automatically "upgrading disk". Do I need to wait until the upgrade is complete before I can assign the old 1TB drive to the array?

Yes.

When I tried to do so, it gave me an "invalid expansion error". So I unassigned it and left the new 2TB "upgrading".

Wait for the re-construction on the replacement drive to complete before doing ANYTHING else that changes the disk configuration.  Basically, you should only change ONE disk at a time in the array.

Ok, the re-build is going on but it looks like it is going terribly slow. Here is what the info page looks like:

 

Ok, the re-build is going on but it looks like it is going terribly slow. Here is what the info page looks like:

 

57MB/s is a very good speed.  It is not slow at all.

Ok, I was so use to seeing speeds over 100MB/sec.