Since I upgraded to Garmin Express I can't upload treadmill runs. It appears from watching the sync process that a single treadmill run transfers to the Mac as many, many files (like hundreds).
Update - today the upload worked. The first attempt crashed the watch - it just shut off. After that, it took much longer than normal for the watch to boot up. The upload then started - it took about an hour and uploaded 3 treadmill runs and one outdoor run as 62 files. Despite uploading it as 62 files, the data is correct on the Connect site.