This item is from the Yet Another Enhanced IDE/Fast-ATA/ATA-2 FAQ, by John Wehman and Peter den Haan with numerous contributions by others. (v1.92).
Security mode implements a simple password protection scheme. Security can affect just write operations, or both reads and writes. Non-data operations (such as Identify Device) can always be executed regardless of the security status.
There are two security levels: High and Maximum. If the user password is lost and High level security is set, the drive can still be unlocked with the Master password. At Maximum level, there is no way to unlock the drive without erasing all user data.
After a number of incorrect passwords the drive will reject further passwords until a powerdown or hard reset. This makes guessing the password by brute force very difficult.