Aren't mapped drives particular to the user that is running the net
command?
Your asp.net page usually runs under a different identity to the current user's. This means that your code might possibly map the network path successfully the first time you run it. Then when you run it the second time you will get that error.
But then you are looking in vain to delete the mapping for that letter in your cmd
window since the drive was not mapped via your user identity (under which your cmd
window is running after all)
Then there's also the permissions needed to run the net
command, although that might have thrown a different error.