Check the permission settings on the mount point with the partition mounted. Use a terminal to execute the following command.
ls -lhd /mountpoint
Then check to see if your account falls into one of the settings allowed to access the directory.
I notice, for example, that my Ubuntu 10.04 system does not put users into the users group by default so if a mount point was owned by root:users and had permissions rwxrwx--- then I would not have access to it under Ubuntu but I would have access to it under other distributions.