Following up on my last:

I was able to get both the Rino 650 and 655t back in operation by doing the following:

1) Installing GarminCure3 on my computer.

2) Connecting the devices in preboot mode.

3) Noting the software version installed in each device as detected by GarminCure3 in a firmware cure install.

4) Tracking down a previous version of the firmware here: [Only registered and activated users can see links. ]

5) Using GarminCure3 to install on the devices a previous version of software than what was already on the device. This overwrites the corrupted update causing the problem.

6) With each device now recovered and recognized via USB, updating the software to the current version (as of this writing, 5.10).

Voila. Best of luck to you folks, and a hearty thank you to Kunix.