This has happened to me twice now: I plug in the USB stick for my FR70 and no data upload notification occurs. I check my Garmin Connect account and the workout isn't there, then I check my watch and it has no activities (it's set to remove activities after uploading them, so I don't have to manually do so all the time).
The one thing I noticed this time that may or may not have also happened the first time is that I had to log into Garmin Connect when I went to check on it. I'm not sure how often I have to log in, maybe once a month? So, I'm guessing there's a bug that loses data in the upload process when the stored login session is no longer active.
Dan