I recently upgraded to 6x and I think I didn't overwrite my old backup of unraid with the more recent one.  Between my last backup of my config directory and this one, I had added a new disk. 

 

Now, my newest disk is no longer on the list of devices in my array.  If I try to select it again, it shows up as a "new device, not in an array".  If I start my array, any file from that disk is completely missing, even though it is presumably on the parity disk.  If I manually mount the disk  to /mnt/test/ all the files are still on the disk.  Is it possible to manually re-add the disk to the array without having to wipe it and rebuild from parity?

Use the New Config Utility and assign the desired drives. I suggest leaving the "Parity is good" box unchecked.