I'd start with the Live disc that works and have a look at /etc/samba/smb.conf, have a look at what users and groups you have, /etc/groups, /etc/samba/smbpasswd. Some other things to check, what file system is the NAS? Could be that the live disc has those things setup "automagicially" and you'll need to set them up manually on a real install.
Good luck. ;-)