Welcome to the community, hanz7000!
Since the drive spins up but you can't access it, then it should be recognized somewhere. I'd recommend you to check Disk Management and see if the drive appears there (it will show you what is the file system as well). While at it, you should right-click on the volume of your external HDD and check what options you are given. If you can "Change drive letters & Paths, you should try assigning a different drive letter. Here's a tutorial that might help you with that, too:
http://products.wdc.com/support/kb.ashx?id=E51Aa0
Another thing you should check is Device Manager and see if it's properly detected as a device there. You can even try uninstalling and re-installing it from there (
http://technet.microsoft.com/en-us/library/cc725782.aspx ).
Hope these help. Keep us posted!

SuperSoph_WD