In new versions of Ubuntu, USB disks are mounted at /media/YOUR_USERNAME/DISK_NAME and not accessible by other users.

This is great for security, but if you want your disks mounted at /media/DISK_NAME and accessible to all users, you can add a rule to udisks like so:

echo 'ENV{ID_FS_USAGE}=="filesystem", ENV{UDISKS_FILESYSTEM_SHARED}="1"' | \
sudo tee -a /etc/udev/rules.d/99-udisks2.rules