MTP Devices and the Mac

More and more Garmin devices are released that only support the Media Transfer Protocol (MTP), like the Zũmo XT, and most recent Camper and Drive(Smart) models, but also various fitness and training watches. Apple does support MTP, but not optimal. Only one program can simultaneously communicate with the device, to other programs the device is not discoverable while the first program is active.
Particularly Garmin Express can be a problem because it is setup by default to start automatically. Therefore adjust the Express settings. Uncheck in "General" "Always keep Garmin Express running in the background." and "Automatically launch Garmin Express when a Garmin device is connected.".

And then there is another MTP-problem: The device is not visible in Finder... This can be solved by using the program Android File Transfer. This program will enable access to the files on the device.

However Android File Transfer comes with it's own problem: It gets launched automatically upon connecting an MTP-device to the Mac and the program has no preferences menu to disable that.
But this can also be solved by following these steps:

Close Android File Transfer should it still be active.
Start "Activity Monitor" (It can be found under "Utilities")
End the "Android File Transfer Agent" process.
Open "System preferences" and go to "Users and Groups"
On the left select your account and click on "Login Items"
Select the item "Android File Transfer Agent" and click the minus sign at the bottom to delete it.
Start Finder.
Hold the Option/Alt key.
Choose in the menu "Go" > "Library"
Go to the folder Application Support/Google/Android File Transfer.
Delete "Android File Transfer Agent".
Go to the folder "Applications", hold the Control key and click on "Android File Transfer" (Right click is also an option if your mouse is setup accordingly) and select in the popup menu "Show Package Contents"
Go to the folder Contents/Helpers.
Delete "Android File Transfer Agent" here as well.
Empty Trash as a precaution.