Problems mounting NTFS HDD’s are common (commonly an external HDD used on a windows system before Ubuntu) where the disc has not been unmounted properly during the last session of use (read as “safely removed” if previously used on a Windows system).
STEP 1: Find out what Ubuntu has named the HDD (commonly sdb1 where the primary HDD is sda1)
sudo fdisk -l
STEP 2: Use the name of this HDD by running the NTFSFIX code below, replacing Z for the letter assigned by Ubuntu.
sudo ntfsfix /dev/sdZ1
The external HDD should now mount and be fully operational.
Posted by Mr. Todd 
The Ubuntu Dojo 