You're off topic for this forum, which is dedicated to Windows Live OneCare.
Your question may be better posed to another forum:
Vista: http://social.answers.microsoft.com/Forums/en-US/categories
However, if the drive is not being recognized, I believe that you may have a hardware problem with the drive. If it is not being recognized and visible in Windows Explorer, but the drive is detected at attach, that means that the drive does not appear to Windows to have any data on it and it would require formatting before use.
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator