Yep I have tried three SD cards ( Two SDHC 8GB Samsung Class 6 , One SanDisk Class 3 4GB SDHC) . I doubt they are all faulty
I had been emailing HAL9 , who suggested creating a Stock ( 6.0 Marshmellow ) SD Bootable Card
I attempted this by making two stock Android (6.0) update SD cards using the rockchip upgrade tool.
On one card I enabled the 'SD boot' option
On the second card I enabled the 'upgrade firmware' option
I selected the stock 6.0 image as advised.
I tried both SD cards (one 4GB the other 8GB) , after resetting the stereo , holding the power button until lights flash, I am stuck again for over 20 minutes on a 'navigation system' page with a marshmallow icon , I took picture of this (attached)
I waited 20 minutes for progress but again I am stuck here