Je viens de lire une possible solution sur MacFixIt.com :
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One reader reports a variation on this problem, when he tried to update to Mac OS 10.0.2, it refused. It turns out that the reason was that it was looking for the Essentials.pkg receipt file; without it present it assumed that an update was not okay to do. However, the user had previously deleted this file (not recommended!). He found a work-around of sorts: Open the 10.0.2 package file (via the Show Package Contents contextual menu item) and delete the file in the Resources folder called InstallationCheck. The install should now work. But be careful: only do this if you are confident that you really do have all the updates that the file is looking for; otherwise you may make a bad situation worse!
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Gwenhiver Truly addicted to MacG