The only other possibility, is that your drive already has a (non NULL) password set on it. Without that password, you cannot erase the drive.

Edit: I suppose perhaps only the "master" password could be getting in the way. So you could retry the erase command with a "user" password, though I don't know if it would make any difference:

hdparm --user-master u --security-set-pass NULL /dev/sdX
hdparm --user-master u --security-erase NULL /dev/sdX


-ml