Hi
Usually the HDD password should work after the motherboard replacement.
The point is that the HDD password is stored in a special area of the HDD and it should not be a matter that motherboard has been replaced.
I found this info in Toshiba knowledge base:
TOSHIBA HDD Password summary
http://aps2.toshiba-tro.de/kb0/TSB6B01MC000CR01.htm
What is A HDD Password?
http://aps2.toshiba-tro.de/kb0/FAQ9202JG0000R01.htm
Hope this helps!