can't unmount internal mechanical HD

hi
client is running 2 macpros
10.11.6
2 internal mechanical HDs 1 TB each
ttp 9.5.3

boot from HD1 and then clone to HD2

to effect some HD routines i boot from HD2 [volume rebuild OR volume optimization]
HD1 must be 'unmounted'

got error messages that HD1 COULD NOT BE UNMOUNTED
tried disk util - no go
did some reading online

finally used 'activity monitor' to search 'mds' then 'Finder' [case is important]
the QUIT and FORCE QUIT the processes - the 'mds' and 'mds_store' came back quickly and required FORCE QUIT a few more times

BOTH MACS HAD THE SAME ISSUE AND FIXED THE SAME WAY

BUT
i was able to 'unmount' HD1 and run 'volume rebuild' successfully
i am wondering if moving the to 10.12 might help?
the are an adobe cc shop and required 10.11 for client purposes but they are only now migrating to adobe cc 2017
i am trying to move them to suitcase fusion 8 but interface is quite different