Putting an old HD in a new PS3 without formatting

[ghettoglamour]

Yes people, some of you have told me what would happen. Some other though told me something different. How could I know who really has an idea?

I did the backup after logging in with my account, didn't work either.
Now I have to restore my 500gb HDD in the old PS3...
Then I can shrink the data to 160gb and transfer it to the other console or save the files manually...
But why would the 2nd option work actually? It won't use save files from another PS3 backup, but some random save files in an USB stick are fine to it? WTF??