I get "/tmp is not writeable by the webserver" on migration
If you search Google for "drupal iis tmp not writeable by the webserver", the first result is a post on drupal.org, telling you to:


  Try going to admin/settings/file-system and set /tmp to just tmp


and a comment saying:


  Confirming this fix. It might have something to do with going from linux system to windows.


I guess that will work in your case too.