G'day Howard,
I have an OTG cable (micro USB to USB female) lead that came with my Sony Xperia Z1 Compact smartphone. You can buy such a cable
here, for example.
It allows me to mount any USB pluggable device: hard drive, thumb drive, mouse, keyboard or Garmin to my smartphone. I have used it to mount my Garmin Edge 500 to copy activities files to the phone and tcx files to the Garmin. RWGPS permits an activity file to be emailed and auto uploaded from the email. Does Strava have something similar?
There is no power being supplied via the USB connection, just a data connection in these cases. If you mount an external hard drive for instance, it will only work if it can take an external power source. But my Garmin Edge 500 just mounts like a memory /SD card with all directories and files available for all the things you can do to files and directories on your grown up computer.
This might be useful for those travelling abroad. For example, load the tcx files for each PBP stage onto your smartphone memory card, and copy them to your Garmin as needed. Similarly copy oompleted activity files (.fit) from your Garmin to your phone's memory card. This is more apt for the 500 as it has only modest onboard memory.
Hope that helps
Cheers, Simon