The designations of the drives are assigned dynamically, even from one boot to another they can change. They are assigned as the linux kernel identifies then as they spin up and initialize.
Go by the model/serial numbers. NEVER assume the same disk if all you are going by is sda, sdb, etc.
It sounds as if you still have a disk.cfg file that points to the old disk config.
What version of unRAID are you running?
What version of the preclear script are you running?
Type
preclear_disk.sh -v
to see the version.
I'm using UnRaid 5.0 beta 11 and preclear version is 1.13.
I am sure that sda, sdb etc. is not part of my array, so I know that is not the issue. Can you help me check the disk.cfg file. Thanks so much for your help.