If I'm following correctly, your old drive was previously bootable and did previously prompt for a password on boot. If that is the case, it is almost certain at this point that something has clearly overwritten the main header, and if you have already tried the "Mount partition using system encryption without pre-boot authentication" box without success, then you are probably out of luck. You could also try marking the "Use backup header embedded in volume if available" option at the same time, but if that doesn't work, then since you don't have a recovery disc, you are probably out of luck. That having been said, there is one more thing I haven't suggested. Your new motherboard may be set to EFI/secure boot only and TrueCrypt uses legacy booting. If that is the case and you change the settings so it will boot to legacy systems, then you could try the second test again. At that point, again, assuming this was a bootable encrypted system drive that prompted for a password at boot to begin with, if you are prompted for the password, there may be more you can do. For instance, find a "live" Linux distribution that includes TrueCrypt or something compatible and see if you can mount the drive from there.