With a modern motherboard, there is nothing dangerous whatsoever about flashing the BIOS/firmware. There are plenty of problems that can be fixed by flashing the BIOS when it the checksum is fine, just so that you know.
We already know it is not the HDD as a new one was tried. If the BIOS can't see the HDD, there is no way Windoze will. And the HDD will spin up if it has a power connection, even if there is no data cable connected, so there is no point checking that.
The problem will be either the PSU, motherboard, CPU or BIOS.