OK, my semi-profesional (or not
If I am reading this right (I am somewhat in a hurry, I will re-read later to make sure I don't sound like an idiot
Your initial 80GB drive, although risky, MAY be recoverable via freezing it about 24-36 hours, then taking it out and quickly trying to get as much data off as possible. As soon as the drive warms up, it will die (for good), so get it FAST.
As far as switching circut boards, make sure the drives are 100% exactly the same, and try it again. Trying it in another computer may help also, make sure its the slave though (so you can boot & recover).
Just try to freeeze it, it HAS worked for people; either that, or try DriveSavers (the bill ranges from $200 for a failed recovery to, for 80 GB i think like $2000 max for a 100% full recovery, depending on the type of service (ex economy is the cheapest)).
Hope I was some help!