What is "EFI Boot"?
 

Despite the many reports that WD MyBook drives could not boot an SD Backup of an OS X 10.6 installation via FireWire... I went ahead and gave it "a try". It wasn't too great of a risk as I had a functional SD backup of 10.5.8 on another MyBook drive.

It worked fine... which was a pleasant surprise!

What I'm curious about, however, is the name given to the 10.6 backup was "Full Backup2" --- and that is how it's shown in Finder.

But when I restart the iMac with "option" depressed, this volume is shown as "EFI Boot". Does this have any significance? Why was it changed here but not in Finder... or more important: Why changed at all?

No real significance. We do set the EFI disk name to the name of the drive, and–for reasons we've never been able to figure out–it sometimes doesn't 'take'. But it has no effect on startup.