All the ones where I've seen this is due to the change in the driver structure change in ESXi between 5.1 and 5.5. The new drivers only work if the controller is set to handle EHCI handoff. Whats happening during the boot when EHCI handoff is disabled is that the EHCI drivers start in ESXi and attempt to take over the USB input devices/drives, when this happens the boot level stuff shuts down so nothing is handling the devices because EHCI handoff to the drivers could not occur.