I hooked up my Garmin 510 to my MacBookPro (OSX 10.9.2) after todays ride. As soon as the 510 device was recognized, the Garmin Express updated the 510 to the latest 3.0 firmware. There was no option for me to stop or control the update and it just ran, albeit very quickly, and was done. I went to "Sync" todays ride and it was gone along with all the other ride data. Ugh...I spent an hour on the phone with tech support and they too were baffled to why the update proceeded without an option for me to start/stop at my discretion. Very frustrating...they did recommend I load ride data via teh web from here on out until Garmin Express is fixed.
Best of luck to you all...
Cheers,
Mike