Rafael Garcia-Suarez shares a helpful tip to correctly enable APM in his D420 running Ubuntu, but I can confirm that my D620 behaves the same -ignoring APM level 255 and disabling APM completely instead- and that his fix runs equally well, just by adding
/dev/sda {
apm = 254
}
to /etc/hdparm.conf and making sure that /etc/init.d/hdparm runs at startup (the default after apt-get installing hdparm in my case).