Thanks for the push, I really appreciate it! Unfortunately, I cannot get past the following message from Updater: there are no usb devices available

I have tried with three different Garmin units (nuvi52, gsmap64s, and nuvi57) on two different PCs: Win 10 and Vista). No joy.

EDit: The problem may be my unfamiliarity with preboot. What is preboot, and how to achieve it?

OK I've done a little more research, and now successfully have the device in Mass Storage mode, and believe I sorted out preboot -- thumb on upper left, hit power, then connect to USB before calibration screen, thought I don't see preboot mode anywhere on the display. Still no joy: there are no usb devices available