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does the fact that the HDD from the system with the bad board is already modded? I used the splinter / Linux method for what it's worth.
Did you back up the eeprom? This is great because you can unlock the HDD, provided you backed up the eeprom (to your computer), using XboxHDM.
The problem is still there, though. You need the eeprom from the working motherboards to lock the good HDD (that you modded and *hopefully* backed up the eeprom for) to it. I didn't see if you found out whether the eeprom chips for the various motherboards were compatible, but I would probably just build an eeprom reader, extract the eeprom to your pc, and then unlock the modded HDD with the eeprom you had backed up, to relock it with the eeprom you've now extracted from the good motherboard.
Here's a link to a tut on how to build one, I built one and I don't have a ton of experience in soldering. It's really easy and all of the parts are at your local radioshack.