Your volume is probably encrypted with LUKS, here's how to mount it:

You need:

sudo apt-get install cryptsetup

To decrypt the volume:

sudo cryptsetup luksOpen /dev/sda1 my_encrypted_volume

Now you can mount it as usual:

sudo mkdir /media/my_device
sudo mount /dev/mapper/my_encrypted_volume /media/my_device

To lock the container again, it needs to be unmounted first:

sudo umount /media/my_device
sudo cryptsetup luksClose my_encrypted_volume

To automatically put it in the /media location, use the udisks tool

sudo udisks --mount /dev/mapper/my_encrypted_volume