According to the BIOS Chip datasheet it is possible to prohibit reading from and writing to the chip by connecting pins 7 (#HOLD) to the ground. The trick is to disable the main chip and re-enabling it as soon as the backup BIOS loads. N.B. The chips are rather small and connecting wrong pins may damage the MB.
Each BIOS chip has 8 pins. Pin 1 is marked with blue dot in the left lower corner of each chip. The pins are numbered around the chip, going counterclockwise from the blue dot. See the photo for an example.
The procedure is straight forward:
- With a "multi meter lead" connected anywhere to the GND (Chassi) and the sharp end touching pin 7 of the main BIOS CHIP (M_BIOS in the photo), power up the system. (it is like connecting pin 4(GND) and 7(#HOLD)).
- Wait until the old BIOS shows up, memory test, welcome screen or the DuaBOOT blue screen after the 3 cycles start.
- Then release pin 7. The backup BIOS shall detect that the main BIOS does not match the backup and will re-flash the main BIOS with the backup BIOS automatically.
- The system will be rebooted with the main BIOS.