Comment 286 for bug 59695

to Bart Samwel and all others who are interested :

For some people an apm of 254 doesn't work. For example :
http://ubuntuforums.org/showpost.php?p=3915142&postcount=553

Maybe an apm of 255 works for some of those people.

For others "sudo smartctl -o on /dev/sda" is needed in order to be able to make apm 254 work for some drives (particularly certain samsung drives).
At least two people have confirmed this. For example see http://ubuntuforums.org/showpost.php?p=3672087&postcount=18

From :
https://bugs.launchpad.net/ubuntu/+source/hdparm/+bug/172287/comments/1 :

[quote=Matthias Dietrich]
Looking at hdparm's code, we can see that the value of 255 will NOT be sent to the harddrive, but it will send the "disable APM"(0x85) command instead of "set APM to xxx" (0x05).
[/quote]