I can't think of anything that explains your symptoms ... unless you are using Mint14.
In the olden days if you mounted things to /media/LABEL and changed permissions to 777 everything worked as expected. /media would be at 755, /media/LABEL would be at 777, and everyone and your Aunt Tilly would have access.
Today, if you mount things to /media/$USER/LABEL things get a little complicated because the developers decided to over think the plumbing. /media is still at 755 so there's no change there but take a look at /media/$USER:
$ ls -dl /media/altair
drwxr-x---+ 3 root root 4096 Jan 26 07:29 /media/altair
That doesn't look good does it. At first sight it looks like only root will have access to the directory until you see that "+" at the end. It's using Access Control Lists to determine who has access:
$ getfacl /media/altair
getfacl: Removing leading '/' from absolute path names
# file: media/altair
# owner: root
# group: root
Altair is the only user ( aside from root ) who has permissions to access the directory so you can either add a "force user = altair" to your share definition which will convert the samba client to the only user who can access the contents of the directory or move the mount point to /media/LABEL and restore things to the way it used to be.