hdparm: Unable to change master passsword

[jd1008]

hdparm -I /dev/sdc
.
.
.
.
Security:
         Master password revision code = 65534
                 supported
                 enabled
                 locked
         not     frozen
         not     expired: security count
                 supported: enhanced erase
         Security level high

# hdparm --user-master m --security-unlock thepassword /dev/sdc
security_password="thepassword"

/dev/sdc:
  Issuing SECURITY_UNLOCK command, password="hdparm", user=master
SECURITY_UNLOCK: Input/output error