@jlw

You can connect your Garmin GPS in USB mass storage mode and show the map in BaseCamp or you can use a USB-stick or a memory card. All are USB mass storage mode devices.
Code:
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The GarminDevice.xml file in your Garmin unit gives clues to where different map files and additional files should be stored. BaseCamp should anyway always be able to read "[USB-stick or USB connected Memory Card]:\Garmin\any_file_name.img" (example "X:\Garmin\Gotalandv3.img" where X is the drive letter for a USB mass storage mode device.).

External HDs are not inside the Garmin scope of USB mass storage mode devices. Try this tool if you want a virtual SD card residing on your hard drive: [Only registered and activated users can see links. ]