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I use OS X with a non-admin account, thus when ever I install and upgrade apps i'm asked for the admin username and password. Just now i'm trying to upgrade Thunderbird to version 3.0. Following these steps:

  1. Drag Thunderbird to /Applications it asks me to Authenticate, I press the Authenticate putton
  2. I get a message "An item named Thunderbird allready exists in this location. Do you want to replace it with the one you're moving". That's my old Thunderbird, which should be overwritten so I press the Replace button.
  3. It asks for admin username and password and I enter those.
  4. I get an error message "The operation can't be completed because an item with the name "Thunderbird already exists". (see item 2, i told it replace it, yet it refuses to)
  5. Start from again from 1.

I've had this error for a long time now, but before this Finder has always managed to overwrite the old app on the second time. This is not the end of the world as i can delete the old version and install the new one, but this has been one little annoyance for me for a long time (since 10.5.x), since something clearly is not working.

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If you take a look at the permissions on the application, you'll see that there is either an Access Control List issue, or that your user doesn't have permission to manipulate the file.

I have seen this behavior mostly with new versions of FireFox, where the updater isn't running...

The resolution for this is just as Mark mentioned, trash the original, and then copy it to the target (applications) directory.

I don't have anything that this is occurring with right now, so I can't point to the permissions/acl issue, but that's what I tracked it down to in the past. It happens with both Leopard & Snow Leopard.... But very rarely....

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