Do not confuse the root password, which is valid for the UNIX system (and in most cases stored in the form of a one-way hash in a file named /etc/shadow), with the password used to authenticate against Samba. Samba does not know the UNIX password. Root access to Samba resources requires that a Samba account for root must first be created. This is done with the smbpasswd command as follows:

root#  smbpasswd -a root
New SMB password: secret
Retype new SMB password: secret