Lag-na, thanks for you guidance, but unfortunately I failed to get it to work. Because my share is an HDD (Drobo) attached to the iMac. I couldn’t locate the share name the way you suggested, but I went to the sharing pane in system preferences and found that the name of the iMac is 27-inch-iMac and it can be accessed at 27-inch-iMac.local.
I followed your instructions, entering the Remote Directory path as "27-inch-iMac.local/Drobo/Music. It seemed to work; I got the nice green tick. I had done this on Volumio 1.4 which already had a successful NFS connection to Volumes/Drobo/Music. i.e. It appeared that I had two successful connections to the same music folder by two different methods.
I then reflashed the SD card with Volumio 1.4 so that I had a completely new installation without the NFS connection. Unfortunately, I have had no success with this so it looks like that for now I’ll have to revert to NFS.
If you have any other ideas I’ll like to try them.