Drush does not "see" the installation of Drupal

I have a drupal installation running in a roaming virtual machine. This works well, but when I try to use Drush on this site I get very peculiar behavior. The modules that are listed when I run "drush pm-list" are all from a non-existent Drupal 8 installation. I tried to set –root = and –uri = but in vain. Drush simply ignores my real installation of Drupal 7 and instead generates non-existent modules from a nonexistent installation.