I've been using my Garmin Edge 840 since December last year without any major issues.
Last two weeks became a huge headache.
In July I updated maps using Garmin Express. I noticed I had to chose Eastern Europe region and there is no way for me to have whole Europe like it is on my son's Garmin Edge 530 and my wife's Edge 830.
Two weeks ago I flew with my bicycle for 5 days to Morocco and then 9 days to Tenerife. After landing in Morocco I wanted to connect the Edge 840 to Garmin Express to upload the map of Africa into the unit. At that point I discovered that my Edge 840 would not enter Mass Storage Mode anymore. Each time I connected a data cable the icon of data transfer would appear on my unit's screen for about 5-10 seconds and it would go back to normal mode. Forcing mass storage mode would not make the computer see that the unit was connected. The MacOS system profiler doesn't even recognise that anything is connected. Tried several computers and several cables which previously worked for Garmin Express data connection with my Edge 840.
I could upload the map of Africa using WiFi.
Then one of my activities in Morocco got corrupted. It shows up in my Edge 840, but it does not sync to Garmin Connect. Only that one particular activity.
After I flew to Tenerife, I found out I see only blank islands without roads on the unit's map: Eastern Europe Map. Without Mass Storage Mode and Garmin Express I could not get Western Europe Map which I assume has Canary Islands. It is impossible to delete Eastern Europe and upload Western Europe from the unit itself. I can get any other continent including North America using WiFi connection, just not Western Europe.
It looks like the unit is faulty, but I'm posting this here in a last breath of hope that someone had a similar problem and maybe there is still chance to salvage that one activity file from Morocco that I am missing in my Garmin Connect account.