One more thing :
when i try to change uuid of the file, it seems that it works but not really :
I create a vdi file in virtualbox and install an os. After, I make a copy of this file (with windows explorer, not with vboxmanage).
After, I try to change UUID :

Ok ? No, if I try to use the copy in virtualbox->error : same UUID than a file already registred and if I make a binary compare of the 2 files, they're identical.
So, setvdiuuid seems to work but no. And it is the same with sethduuid.
I don't understand.
Any idea ?