Philippe, this FLASH write problem of yours is getting harder.
I am worried that you have a motherboard with a "secret write enable". If this is the case, you have a few choices. One is to take the romimage and convert it to a flash image that the standard FLASH rewrite tool can handle, and then use the standard tool. The second is to find the "secret write enable" and figure out how it works. If you don't do much hardware, get a hardware friend to help. This is easier work than it sounds.
Are you sure there is not a flash enable jumper on the board?
ron