Garmin Backup runs in your menu bar and waits until you plug in your Garmin or SRM PC8. When it detects your device, Garmin Backup copies any new exercise files to your backup folder. Each device's files are in a separate folder, even if you have more than one Garmin of the same model.
As your Garmin fills up, it deletes older session files to make way for more recent ones. Using Garmin Backup, you can ensure that you have a copy of all your training data.