Ok, this mean that your chip is ok.

You write that if you flash FW 6.60 your PC see your nuvi.

Please try this:

Flash to version 6.60
Clear non vol
Connect nuvi to PC and update nuvi with webupdater to FW 7.90 ( don't use Garmin express)

Let us know if after this your PC recognize nuvi with FW 7.90.