RAW format.

[Admiral Huddy]

Why would an external HDD show all of a sudden as RAW format?

A quick format put this back to NTFS but why would this happen in the first place?

[Daz]

Shafted partition table comes to mind. Could be a failing disk or some software going barney.

[Mark]

Partition type byte in the partition table most likely. Unless you've been playing about with the partition table recently or have a virus, it's most likely a soon-to-be-extinct disk.

[MarcLister]

Gas mark 5 for 40-45 minutes should do the job Huddy. ;D