It was probably formatted as NTFS at some point, right? Do you know? Is it an external drive?
Here's an interesting page:
http://www.z-a-recovery.com/art-raw-filesystem.htmSometimes, the volume fails in such a way that it becomes RAW.
This page implies data is recoverable. The "raw" bit's a good hint, and you'll probably have to use some software to fix it, but you can search for like, "hard drive raw recovery" or something.
Maybe try this software:
http://www.cgsecurity.org/wiki/TestDiskThis one costs money but if TestDisk doesn't work and you're desperate:
http://www.z-a-recovery.com/register.htm If the hard drive isn't dying, and something just got corrupted somehow (any odd shutdowns? power failure? any crashes or anything?), you'll have to re-format the disk to use it again, but you will need to recover your files first because the format will delete them all. You can still recover them but it will be much more difficult and possibly expensive, so try to recover the data before you format it.
But if the drive is dying, and believe me once drives start to die all kinds of weird things start to happen (how old is it btw? less than a year? couple years old?), but if it's actually dying then I wouldn't recommend formatting and using it again, since it's likely to corrupt itself again and it's always bound to be unrecoverable at some point.