The AMI BIOS must support the following three things: USB, USB mass storage, and high-speed USB. Not all AMI customers include this full support in the PCs that they ship. However, if "USB-HDD" is listed as an option it is possible that it supports USB and mass storage. Even without high-speed support, the CY4611 or CY4615 mass storage reference designs should at least enumerate at full speed. One may need to use a bus analyzer, such as CATC, to get a trace of the enumeration sequence.
We offer DOS drivers, which are intended to provide support to end users until BIOS's with full USB support are available as well as to offer support to legacy PCs. One may want to consider this option both for legacy customer support and to help analyze your situation.