Conventional command-line tar apps don't recognize the presence of a tape drive. Tapes in Windows are not treated like removable drives but are a device unto themselves that require special handling. The only way to get around this is to use a tar application that has been specifically written to deal with Windows tape devices. Hamilton Labs (http://www.hamiltonlabs.com/) has one such utility in their Hamilton C Shell. A free evaluation version is available online.
This was first published in November 2003