My Passport 500gb is Unallocated and it can't be Initialized

I have Windows7 (64 bit). I tried to connect an USB external hard drive and the disk shows up in the system tray (Safely Remove the Hardware). But under my computer it will not show. I went to the Computer Management under the Disk Management I could see the disk. But it says, it is Unallocated and it has to be Initialized. I have lot data in the disk. Can I still initialize the disk? Or how can I make the computer to understand my external hard drive without the lose of any data?
Thank you,

I need the data very much so erasing it is not so much of an option if I can.

I have the exact same problem: I can see the device EVERYWHERE but not in the usable hard disks or removable peripherals.

My model is the 750 though, and it’s working: I used it on another computer.

I have Win7 Ultimate 64bit.

Any hint?

Found the problem: due to install half made, it was missing a letter.

Just go to manage computer, disks then assign a letter to your drive!

If the drive shows in Disk Management then maybe TestDisk will fix it.