From the release notes of the latest bios:
Note: For boards that support Intel® AMT and currently have a BIOS version older than 1788, you must first flash to BIOS version 1788 and then flash from 1788 to the newer BIOS version, otherwise the flash update will fail.
I guess that you may try the update in two parts.
All the best,
Thanks for the suggestion--but no luck. It won't boot off USB, hard drive or CD. I guess a new MB is in order.