You said you changed permissions, but did you also change OWNERSHIP?
If not, log in via SSH again, and do this:
MyBookLive:~# cd /shares
Step below: change ((sharename)) to the case-sensitive name of the share
MyBookLive:~# chown nobody.share ((sharename))
MyBookLive:~# cd ((sharename))
Step below: change ((username)) to one of the users that has WRITE access to the share
MyBookLive:~# chown -R ((username)).share *
usng your new user login, create a file in the new share directory using windows or Mac. Then SSH into the drive and observe te permissions and group associated with this file. Next go the old file and change their permissions and group to match what you saw on the new file.