Might be Apple/Microsoft file incompatibility. I once had a Creative Zen MP3 player. Although the Mac would swear it was empty, every time I checked the device it was full of all the old files. The only way to make it work with the Mac (ie, if I wanted to add podcasts or other music) was to reformat it every time and load on a full set of files again. Might be a similar problem with your Garmin. Can you do a reformat and restore, minus your unwanted files?