by using xtreme bootmaker you already make sure that your mtkflash is adapted for your via6421l so no need to get other mtkflash. in xtreme bootmaker you choose the fw you want your drive to be patched with, then choose via6421 in the scrolldown list, click detect i/o and check both boke's patch and bad flash compatibility boxes, then select your usb stick drive letter and check the format and make bootable box. i assume you've already done this?
with your usb stick ready, follow these steps exactly:
x with pc and 360 turned off, hook up your drive to the sata port accessed from the inside of your computer. the via card has two sata ports, you must use the second one, NOT the one accessable from the outside of your computer. with xtreme bootmaker mtkflash is edited only for the inside port.
x with 360 turned OFF, start your pc and boot with usb. when booted and you're in dos, turn your 360 on.
x do the mtkflash r /m orig.bin and hit enter
x now you're to choose port and you should see one port named XTREME. when you see the ports do not yet choose, so BEFORE choosing a number turn your 360 off.
x once turned off, choose the xtreme port, when the port has been chosen, count to 10
x after 10 seconds, turn your 360 back on and it starts dumping right away
this is known as the 10 sec trick and must be performed on all MS28. do the same when flashing. i assume you know that you must extract your key from the orig.bin and put it into the xtreme.bin before flashing.