For a long time I had my HDD go to sleep after 5 minutes, only to find that 'something' urged it to spin up after some time. Then it would sleep again, and so forth.
Then I decided to do it the way KC Furge advised. The disk spins all the time, and the Asus takes about 11.5 Watts all day.
Now I have tested the 'goto sleep after 5 minutes' again, but only with radio off, that means: no WLAN. And with an energy meter I can see that now the Asus keeps sleeping all day long, drowsing a meager 4.6 Watts that way.
So, the WLAN causes all these unwanted spin ups!
Probably because someone scans the Wifi signals or tries to get in.
I have TKIP/AES and WPA-PSK/WPA2-PSK configured, and as you might know the Asus keeps the PSK on the disk, so if I connect to it wirelessly it will also spin up the disk. My password comes from https://www.grc.com/passwords.htm
Now I will try to make the copy button start/stop the WLAN, to have the best of both worlds: an almost cold Asus that only spins the disk when absolutely necessary.
Update:
I now know how to avoid spinning up the HD with WLAN on. The secret is a secret SSID... It is not enough to hide the SSID, no, you have to use a 'secret' SSID that you did not use before and made public. The wireless devices in the neighbourhood all keep internal lists of SSID's they once made contact with, and they will always try to renew the contact, even if your SSID is hidden.
So, to avoid this, you have to choose a new SSID and keep (i said KEEP) it hidden from the start. Then no other device will see it, nor has it's name in his list of former engagements. Bingo!
Now my Asus is quiet all day. I won't tell you what SSID I have chosen....
I provide this information in the hope it will prove useful for someone.
Regards,
Marc