I can't necessarily say I have solved this problem. But, it is "fixed" for now on my machine. I have a multi-boot Win7, multi-partition system with SL, Lion and Mt Lion and installer partitions running. I started to get this EBIOS error everything. First, I reset the mb bios, then turned off/on the legacy usb with differnet boots. Nothing.
Then I noticed I had a Canon scanner/printer usb plugged in at boot and showing up in the boot setup list. It had been plugged in and "on" when I tested a installer usb. That is when it all started for me.
I turned off the printer and all devices and the EBIOS boot error went away. Simple as that... I suspect this has something to do with non-bootable usb devices being plugged in at boot. Thought I'd share.