MD1220 (on an MD3220i head) failed disk copy back function not working

Can anyone advise: 

I had a disk failure on a MD1220 shelf, attached to a 3220i head unit.  Raid 10 pool.   The RAID as expected utilised a hot spare on the head unit (ignored the spare sitting on the same shelf!). 

Disk replacement would normally kick off a copy-back function. 

In this case,  the replaced disk is listing a status of 'Optimal' / Unassigned.  Hot spare still in use. 

If I select to 'replace' the hot spare, it tells me this cannot be done.  If I do the same on the replaced disk, it's listing the Physical Disk Status as 'missing'. 

Any ideas?