The master password is not used by the xbox. It uses the user password to unlock the disk.
You could use the windows liveinfo beta 3 program to check your eeprom.bin file. It should even give you the correct user locking password if you select the correct disk. You can also check the serial number in the eeprom.bin file and compare it with your xbox eeprom number.
It is not 100% clear to me where xboxhdm searches for the eeprom.bin file.
I usually copy it to a floppy disk. That way, I can use my xboxhdm cd for different xboxes.
You can use the dos tools and type in the password directly. Just make sure you don't make a typo, as you
can end up with a totally unlockable disk. (If the master password is not set or unknown)
regards.