step3

my home-made 'command line interface' via the serial port on the arduino lets me spin up or down any drive I want.

 

this capture shows the system accepting the user's request to take drive 2 down and it prints a summary of all drives (as a standard reply to any command, actually).

 

you can see the 'd' in the 2nd entry; showing this drive is now powered off or 'down'.

 

Done