Take the card out and try it again
This is what Garmin says in the manual"
The device does not appear as a portable device or as a removable drive or volume on my computer
Disconnect the USB cable from the computer.
Switch off the device.
Connect the USB cable to your device and to a USB port on your computer.
TIP: The USB cable should be connected to a USB port on your computer and not to a USB hub.
The device will automatically turn on and switch to MTP mode or USB mass storage mode. Then an image of the device connected to a computer appears on the screen of the device.
NOTE: If several network drives are connected to the computer, Windows may have problems assigning drive letters to your Garmin
drives. For more information about assigning drive letters, see the Help file for your operating system.