Since about 8 to 10 days ago, my wife and I have both been having difficulties uploading new activities from our 310XT's using Garmin Express. Was there a change in the software/firmware that would cause this on both of our devices at nearly the same time? We try the following until one works. As it turns out, we've had to always get to the last step, the hard reset, before the problem is corrected!
1) remove and replace the usb stick (not sure if 910 requires one)
2) power the device off and then on again
3) Add a new "dummy" activity by simply running the timer for a few seconds.
4) soft reset the device
5) set the device to "Transfer All" under the History settings.
6) delete any old, unwanted activities from history on the device
7) hard reset the device (factory reset) - thankfully, this seems to not delete history, but only personal preferences.