I think the question is how did you mount the other partitions.
** Did you add entries in fstab to mount these partitions? If so find out what the permissions are on them to determine if "other" can access the path to the shared folder.
** If you are mounting them through Nautilus or whatever Ubuntu calls the panel thingy ( sorry I use XFCE - old school I'm afraid ) then I may have an explanation.
I don't know if this comes from Debian, Ubuntu, or Gnome but someone is playing around with something they don't fully understand again. Instead of mounting at /media/LABEL the way they used to they now mount at /media/$USER/LABEL with funky permissions. For example:
You will see this for /media/username
THe little "+" at the end indicates that it's using access control lists to make something that was very simple to work with something that is now complicated to work with.
drwxr-x---+ 3 root root 4096 Nov 8 06:49 username
The only person who is going to be able to access the partition mounted within /media/username is "username" which leaves a remote samba guest out of the loop. You might be temped to just overwrite the permissions to somethig like 777 but it's not clear to me why they made the change or what repercussions changing permissions would have.
You have 2 choices:
 Mount the partitions though fstab so you can take adult control over this process.
 Add an entry into /etc/samba/smb.conf that makes all remote users appear to be you for these samba shares. For example:
Add it to the [global] section - under the workgroup line in smb.conf.
force user = morbius
Then restart samba:
sudo service smbd restart