Hi Snow,
found this in another forum, so seems to work without System.kext as well but there are some sidekicks probably:

>The system.kext can be added later, it is just for the bom.archiever and
>the diskimagemounter to work. Run diskutility after replacing it in System/
>Library/Extensions.

So why is it working for you? Single Prozessor? Did you apply time machinge update before? SMBIOS version?
It seems no one could find out the issue why it sometimes works and sometimes it won't.

P