How to find out which system component disabled the hibernation? (Windows 7)

Use the registry to enable hibernation back.

1. start regedit.exe
2. go to the key HKEY_LOCAL_MACHINE\SYSTEM\CurrentCongtrolSet\Control\Power
3. change the value HibernateEnabled from 0 to 1.

Now reboot and look if you have hibernation back.

I finally got it. Windows 7 and 8 does not allow hibernation if it is installed inside a VHD.

Windows 7 powercfg.exe does not make it explicit. Windows 8 does, a screen shot below:

I cannot figure out the reason why Microsoft impose such limitation. If he puts hiberfil.sys onto a partition from a real disk , things should be able to go smooth.

BUT, is there a way to know it on Windows 7, through a system log or something?