How does it show in “Disk Management”?
Your situation happens sometimes. You could try to assign a new letter to this drive in “Disk Management” like this:
http://technet.microsoft.com/en-gb/library/cc757491%28v=ws.10%29.aspx
If it doesn’t work, you perhaps have no choice but to format it for the future use.
However, in order to rescue all your data there, you could try
a free hard drive recovery tool that has ever completely restored the inaccessible data of my friend back from a mistakenly formatted drive. That ‘s why I recommend it to you.
You could give it a shot here:
http://freeware-fix.blogspot.com/2013/02/external-drive-not-detected.html
Important note: You should not save new file on this “not formatted ” hard drive in case of data loss.
You should format this drive before you have surely restored all your data back.
In case of data loss, your best bet is to keep everything important backed up on at least two different drives.
Hope this works for you.