The external drive will mount with the local login user as owner and read / write permissions set to him only. One way around this problem is to make all remote users look like that local login user for that share:
Edit smb.conf as root:
Add the following line to the share definition section or the [global] section is you're using Nautilus-share:
gksu gedit /etc/samba/smb.conf
Change morbius to your local login user name.
force user = morbius
Save the file, exit gedit, and back in the terminal restart samba:
The "force user" will act as a mask and convert the remote user to you for those shares.
sudo service smbd restart