I was upgrading from Tiger to Snow Leopard (I asked a worker in the Apple store and he assured me it was okay to do so) and about 15 minutes into the installation I got a message saying "Unable to install. Please retry". Then when I retried it says "The contents of this disk can't be changed. Mac OS X couldn't be installed on this disk." I have a black MBP and it is Intel based and I fit the system requirements. Anyone have any ideas? Thanks.