A disk has developed reallocated sectors and I like to check if it will increase the reallocated sector count after a few preclear runs. Can I just unassign the empty disk and then run a preclear on it, without losing parity or should I set a new config anyhow? 

Edited March 6, 20179 yr by dikkiedirk

Once you pre-clear the disk, you would then add it back to the array (assuming it passes).  At that point it will want to rebuild from parity.

I may have misunderstood your question, but to pre-clear a disk that is already assigned to the array, stop the array and unassign the disk.  More than likely you will need to start the array with the disk unassigned for the pre-clear script to run because disk assignments don't get updated until the array is started.  When the pre-clear is finished, re-assign the disk and let it rebuild the disk from parity (even though the rebuild will be of an empty disk). That's how I would do it.  I suppose if you want to maintain parity through the entire process, you can shrink the array first since the disk is empty.