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):
hdparm -B 255 /dev/sda
3) copy this file to 3 locations:
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.