I run Mac OS mavericks and Garmin Express is not recognizing my Edge 500. The old Garmin connect website detects the device, so I suppose it is a problem with the software.
Please see the previous post from (Albert). He indicated on the firmware of an Edge 500 having more than 3 digits. According to you GarminDevice.xml file you are having the exact issue he was describing.
-<Model>
<PartNumber/>
<SoftwareVersion>40863</SoftwareVersion>
<Description>Edge 500</Description>
</Model>
<Id>388*******</Id>
-<MassStorageMode>
-<DataType>
<Name>FitnessCourses</Name>
Please follow the steps given by Albert on how to correct this issue.