There are a few instances in which a master reset may want to be performed on the Edge 305 or 205, such as:
All settings, workouts, courses, and satellite data may be erased when resetting the GPS. Workouts can be backed up to such training programs like Garmin Connect.
To perform the Master Reset:
The Edge 305 or 205 is now reset. Leave the device outside with a clear view of the sky for a minimum of 20 minutes to acquire satellite data.
*If the device is not receiving a satellite signal, perform an AutoLocate before performing a master reset.
†If the device will not power off, try a soft reset.
Garmin Connect - http://connect.garmin.com/?s=1