The "m" designation for the i9250m is for a Galaxy Nexus for the Canadian market. Spec-wise, it is the same as the i9250.
Your desktop might not have the latest drivers to read your Galaxy Nexus' internal storage.You can go to the Samsung Download Center to find the appropriate driver for your phone (see Related Links), and type in your particular model number (ex: I9250). Another alternative is to use the app AirDroid (available in the Google Play Store) and a connected desktop or laptop, which you can see your phone's files via wifi.