Removing the ATA master password
Solution 1:
MHDD can do this with its DISPWD (disable password) command. You have to unlock it first with the UNLOCK command.
Solution 2:
You cannot disable the master password. There is even one set when the drive is manufactured. You can at most set it to empty with NULL in hdparm (see man page for details), though you might not want to try because it can brick the drive.
DISPWD in MHDD does exactly what --security-disable in hdparm does, that is disabling the user password, which will be done after a successful security erase, as mentioned in the OP.