sometimes especially external disks get “lost” using windows. The Windows Disk Tools mostly are not able to recover such a disk and suggest to format it as it isn't formatet. but in many cases the only fault is a corrupt superblock or an inconsistent FAT.
So an approach for recovering – besides professional tools like kroll ontrack – is to use the linux tools for NTFS:
sudo apt-get install ntfs-3g. ntfsfix /dev/hda6