> hdparm -I /dev/sd?

...
Master password revision code 1
     supported
     enabled
     locked
not  frozen
not  expired: security count
...

> hdparm --user-master u --security-unlock PASSWORD /dev/sd?

Issuing SECURITY_UNLOCK command, password="PASSWORD", user=user
SECURITY_ERASE: Input/output error

> hdparm --user-master m --security-unlock PASSWORD /dev/sd?

Issuing SECURITY_UNLOCK command, password="PASSWORD", user=master
SECURITY_ERASE: Input/output error