Have you tried using Disk Utility to verify the filesystem on the 510? If it come up with an error, you might try repairing it if you feel adventurous (but if it were me, I'd back it up first by copying everything on it). OS X knows how to verify/repair the FAT filesystem that Garmin devices come with.
It doesn't even show up on drive so not sure disk utility would do it since computer not recognizing it's there.
It does not look like Garmin is interested at all to help. I have even logged a call about this but with no response. It is very clear that their WebUpdater is causing this issue on Mac computers. I am now sitting with a Edge 510 that I might as well use as a paper weight!
Unfortunately this did not work. I decided to take it in to Garmin so that they can help but after a week they came to the conclusion that the unit can't be rectified. They had to replace the unit with a new unit. Luckily it was still under warranty so I feel sorry for the people out there that have the same issue where the warranty is not valid anymore. So do not try and update firmware if you are using WEbExpress on a MAC!
This did not work for me. I am running the update through wizard on windows. when I went to run the update the update wizard said the file was corrupt.
