The full disk encryption uses LUKS. I have an external drive that is LUKS encrypted and below is how I go about accessing it, I'd imagine the same would work for an additional internal disk. I use this on a server install, so excuse the all command line approach.
You will need to know the device name and partition number of the encrypted partition e.g /dev/sdb2 etc. for the drive. Substitute your device name as required in the below.
Think of a suitable name for the mountpoint for your disk, in this example I use "securetoshiba".
Where you see "georges2051" in the below, substitute your actual username on the machine you are on.
1. Create and take ownership of the mount point:
2. Add an fstab entry for the device:
sudo mkdir /media/securetoshiba
sudo chown -R georges2051:georges2051 /media/securetoshiba
.... add these lines to the end then save and exit the editor:
sudo nano /etc/fstab
If your encrypted drive is in a different format than ext4 then amend the above line accordingly.
# LUKS ext4 on toshiba usb drive
/dev/mapper/securetoshiba /media/securetoshiba ext4 defaults,noauto 0 1
3. Create a couple of simple scripts for mounting and unmounting using whatever editor you like:
## mount securetoshiba
echo "Opening LUKS mapping"
sudo cryptsetup luksOpen /dev/sdb1 securetoshiba
echo "Mounting to /media/securetoshiba"
sudo mount /media/securetoshiba
Set the properties of the two script files to be executable either using your GUI file explorer or using the chmod +x command.
## unmount securetoshiba
echo "Unmounting /media/securetoshiba"
sudo umount /media/securetoshiba
echo "Closing LUKS mapping"
sudo cryptsetup luksClose securetoshiba
Then just run the mount script - you will first be prompted for your sudo password and then for the LUKS encryption passphrase. If successful your unencrypted files should now be accessible via /media/securetoshiba
Just run the umount.sh script when you have finished with the encrypted drive.
I don't claim that the above is a particularly slick way of achieving this, but it works for me