The dd zero command we have not used yet but I would like to try your suggestion. Your thinking about the bit is as good as any at this stage and I would like to follow through a little bit later.
The planned next step will overwrite the first 446 characters, leaving the partition table unchanged. Replace the Grub boot code and install ms bootcode instead. Enable the second primary partition boot flag only, the second partition is where the Linux install is. Install Grub to the second partition boot sector. Then start the system that way.
As I see it using the ms boot code will also prove that the port will respond properly to boot instructions, if that fails I have only one more item up my sleeve at this stage. However I do not expect it will fail as the concept works here. What is being implemented is Old-Polack's experience with hard drives (see his comment earlier) but I have proved here that it work fine on USB as well. I should clarify it works on my system and my USB and one of your liveUSB install on my problem USB key
(that key is not a problem if preformatted ext3).
After having played with the running system we can try your suggestion. Of course there is also the possibility I will be wrong once more !!
There will be a bit more in this thread.