I tried a clean install on DSM 5 with last update. I have the same problem as you, the hhds go to hibernation but they wake up after some time. I have not any package installed, so it has nothing to do with some application trying to access the disks. The only service I have enabled is the DDNS. What bios do you have? I have the latest of HP, not the modded one. Do you think this is the problem?