For the end, i owe you feedback of tried methods , to summarize , unfortunately no good news , it's seems on my problematic nuvi 765, flash chip is dead, like kunix supposed, so i'll have to replace it... If someone knows where could be find flash disk for nuvi 765 for good price, please post it?

All of you thanks for great support, specially kunix and lolly, without their ideas and solutions, wouldn't be possible to try anything ! Now we have one more expirience, when these symptoms occur on 765 nuvi, that means in most of cases, if we tried all know methods, that flash-chip is officially dead !