Problem Adding Physical Drive to VM in Hyper-V

When trying to add the physical CD/DVD Drive to a VM in Hyper-V Manager settings. I have been getting the following error message:

Problem Adding Physical Drive
Error Applying DVD Drive Changes
Failed to add device 'Microsoft Virtual CD/DVD Drive'
Failed to open attachment 'D:'. Error: 'The process cannot access the file
because it is being used by another process.'

The solution?
I had another Virtual Machine that already had the physical CD/DVD Drive associated with it.
Not surprisingly this is what was causing the problem - the clue was in the last part
of the error message.
Sure enough, when I changed the setting of the machine that already had the physical CD/DVD drive to
'None' I could assign the physical drive to the other VM.