I did a "Clear Nonvol" and "Erase all user data" but that didn't change the boot process (but did no harm either).

I then used a variable resistor to change the voltage on the ID pin to see if I could get it to go into the normal power-connected boot mode where the green progress bar goes across the screen under the Garmin logo. When the "USB ID Volts" was below 2.43 the green progress bar did not appear and it went straight from the Garmin logo to the Nuvi copyright page (same as when the ID pin is grounded). When the USB ID voltage as above 2.48 it showed a green progress bar under the Garmin logo for a few seconds which was interrupted by the "waiting for computer" icon (same as leaving the ID pin open). So I suspect that there is no ID pin voltage that will cause the Nuvi go through its boot process without waiting for a USB connection. Perhaps the Garmin power supplies short the USB data pins in addition to having a pull-down resistor on the ID pin. That's something I can't do since the USB data pin traces were lifted off the PCB with the connector.

So... I think I'm going to declare the problem solved since the unit does everything it's supposed to and further attempts to update the firmware could just get me into more trouble.

Thanks again for all the help.