If there is an option in the bios for usb booting, I have not found it. This is probably something that is unique to the emachine unit as it does not have a compact flash socket. AFAIK, the IA-1 only lets you boot from 4 potential sources: Each drive on each IDE controller.
That is, the A: drive (what is that???), the internal sandisk and the external compact flash. There does not appear to be a 4th drive.
My IA-1 does not boot from an external USB HD, and the unit's light does not come on until partially through the win98 boot sequence.
The compact flash socket is located on the right side of the unit and fortunately has an ejector mechanism.