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