OK. I've sorted it.
My BIOS, even when USB is disabled, locates the storage devices on my Printer/Scanner/Card reader. This in turn re-enables the USB settings in BIOS. Therefore USB is still active. Once I physically disconnected the printer, AROS booted fine from disk.
Still trying to boot from CD but that is another matter entirely.
Thanks to everyone for their help. I hope the advise, above, proves useful to someone.