Here is how I permanently fixed it:

1) make a file named “99-hdd-spin-fix.sh”. The important thing is starting with “99″.

2) make sure the file contains the following 2 lines (fix it if you have PATA HDD):

#!/bin/sh
hdparm -B 255 /dev/sda

3) copy this file to 3 locations:
/etc/acpi/suspend.d/
/etc/acpi/resume.d/
/etc/acpi/start.d/

Voila! After that the HDD never spins down on power (looks like it actually spins down on battery at modest rate). Sorry if the instruction is too detailed, no offense.