I've run into the same problem too. Are USB drivers under DOS related to the motherboard? My motherboard is SOLTEK SL-85ERV.
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The following is quoted from shenlan's post at 2003-12-23 20:27:26:
Using USB devices under DOS is very troublesome, because different USB devices need different drivers, and it may even vary with different motherboards. I can use MOS71 to boot a USB flash drive, but I just still can't boot a USB hard drive. You can try a few USB drivers selectively and see which one supports your device; this can't be generalized. It took me a long time to figure it out back then. Now I use a WINPE CD, which completely solves the USB device support problem.