I have a desktop ASUS running Windows 7 Pro and I fortunately download all of my important files to a WD external HD.  A few weeks back my computer crashed and I had to use the System IMAGE Disks to get it up and running again. (Thank you Computer Commando!)  ANYWAY,
I plugged in my external HD and the computer now wants to reformat the external HD, and none of my files are showing up.  They do show up on my laptop running VISTA, but not on the desktop computer.  How can I get the Windows 7 to recognize that there are files on the external HD and that I definitely do not want to reformat it?  Thank youGood question. I have had things like that happen and NEVER found a real clear absolute answer. I think it has something to do with what kind of adapter you have for the external drive. Or the quality of the cable.
You can:
  Tray another USB cable.
  Try another USB adapter.

If that fails, you can link the two computer via Ethernet and transfer files. Not as fast, but it is very reliable.

BTW: Newer laptop computers have an external SATA port . Look for it when you are in the market for a new laptop.
Thank you for your input.  I rummaged around and found another shorter cable to use that was labeled as 2.0, plugged it in and the computer recognized the external HD and all of its files.  I'd had a cable problem before, CHANGED cable,s and everything worked fine, but since I had to do the System Image Recovery, it didn't want to recognize the ext. HD with that same cable.  Go figure?  Thanks again.