While the Garmin 300 jives very nicely when booting natively into a bootcamp installation of Windows Vista, communication fails with Parallels' default guest USB configuration.
Out of the box, Parallels prompts the user for action concerning connected USB devices. Choosing the "Garmin International - Composite" device from the list of connected devices produces an error message:USB device you are trying to connect to the virtual machine is being used by another application. Close that application or unplug the device and plug it again. Then try to connect the device again.To avoid this error message, change the default USB configuration from requesting user input to automatically connecting USB devices to the guest operating system.
- Disconnect the GPS device from the Mac if it is already connected. Shutdown the virtual machine if it is already running.
- Open Parallels Desktop without starting the virtual machine.
Launch Spotlight with Apple + Spacebar, type Parallels Desktop.app, and press Return.
Edit the USB configuration for the appropriate virtual machine.
Click USB Controller from the list of configuration options.- Configure Parallels to automatically connect USB devices to the guest operating system.
Choose Connect to guest OS from the USB Connection Options drop down menu. - Close the virtual machine preferences window by clicking OK; start the virtual machine.
- Once the virtual machine loads, the Parallels Tools initialize, and all booting and login activity complete, connect the Garmin GPS device to the Mac via a USB cable.
- Wait as the Garmin GPS device saves all waypoints, routes, and trip log information to GPX format.
- Windows Vista prompts you with an Autoplay dialogue if the configuration is proper. Otherwise, you will see the same error message as before.

