Even if you diligently followed all the instructions in this book, you might encounter problems while upgrading. I can’t anticipate every difficulty you may have (or provide solutions in every case), but here are some of the most likely issues and how to deal with them.
If the Mountain Lion installer icon has a slash through it, or if you try to run it and it quits immediately, the three likely causes are:
•You’re trying to run it on a Mac whose CPU is incompatible with Mountain Lion (see Model Support for full details). Solution: Buy a new Mac. (Sorry.)
•You’re trying to run it on a Mac that is booted under Leopard or Tiger. Solution: Boot from a Snow Leopard or Lion volume (if ...