Hello Stefan.
You have to look here: http://forum.ixbt.com/topic.cgi?id=11:38403-27 , the Man called "Memphis" made a perfect "ROM" and "Writer" for VIA6421A based controllers. There are a lots of HDD utilities included and ROM code was excellent modified. So I use it now, and actively participate in testing of new releases. The forum is Russian and language Russian, but if You have some technical knowledge, could be fine. He found a lots lots lots of mistakes and just stupid things in VIA original BIOS. There was a lot of complains from users for Your W39V040B Flash chip, as it known as a broken/malfunctioning/low-cost in the professional world, this chip can't be flashed on board of VIA6421A. I was happy and lucky, the W39V040C is good and do not have any problems and it flashed easily not like "B".