Hi and welcome to the forums.
I have the Garmin 62s, but maybe the following procedure may help, if I understand your question correctly...
1) Delete all gpx files in the garmin/gpx directory ( back up any that you want to keep )
2) Unplug usb from PC
3) Reboot GPS. wait for it to fully load the map screen.
4) Reconnect usb
5) Load the new gpx files to the garmin/gpx directory and copy across those that you saved and want in the gps
6) Unplug usb from PC
7) Turn on the gps and view all details in the gpx files incl new caches.
I'm not 100% sure this applies to the Oregon 300, but I had the same issue with the 62s and the above procedure worked for me.
Hope this helps.
Steve
Steve,
Thanks _ I'll try it this evening.
Ray