Ok. Now, I'm a little frustrated.
I replaced the drive. The rebuild completed. I started a parity check... And now, the new drive is disabled before parity could finish.
Come to think of it, the first drive "failed" during a parity check.
It seems the port (cabling?) is the problem, not the drive.
UPDATE:
I have other slots I could use so I stopped the server, shutdown and moved the drive to another slot. I didn't unassign the drive prior to shutting down, because it was already unassigned.
After restarting, the server recognized the drive in the different slot and automatically re-assigned it as disk10 (its prior assignment). The GUI even says that the "configuration is valid".
But, when I start the server disk10 has a status of "Device is Disabled, Contents Emulated".
I'm not sure what I should do. It's not accepting the drive and it's not rebuilding it either.