> Flashtool advertises against using it directly from a laptop,> do you know if it can cause issues with the T420 ? Or if, were
> the bios bricked, I could unbrick it by flashing again externally ?
At the end of the T420 wiki page, it says using flashrom's internal programmer is possible. The laptop warning does not apply to all laptops (depends on how hardware is connected), and the T420 is safe to use in this sense.
If you happen to flash a non-booting coreboot, you can use an external programmer. I am not sure whether the flash chip is powered, though.
Regards,
Angel Pons