I have seen this happen for 3 different Mac users in the last 2 weeks so it appears to be something related to the Mac.

The only way to fix it is to reformat the Edge drive. Disconnect the Edge from the Mac and then the Edge will rebuild the file system.

On the Mac you’d need to use the built-in app called “Disk Utility”.

You can select the existing partition and choose the Erase tab, or select the physical disk and choose the partition tab.

The only option you’d be given for format is “MS-DOS (FAT)”. I’m pretty sure this creates a FAT-32 partition. It is my understanding that some (all?) devices will then re-do the formatting to create the correct FAT-16 partition that they need.

If you reformat the drive on the Edge then the O/S will rebuild the file system on it.

Connect the Edge to a PC. Right click on the Garmin device and select format.

Choose FAT as the file format and deselect the "quick format" option. (See details above for formatting the Edge using a Mac)

After the reformatting is complete dismount the Edge from the PC and switch it on. The Edge will then rebuild the file system.