external hard drive solves the problem. First make a full time machine backup, then install the new software on your computer.
Next you use the migration assistent to move data from the external drive into the new system.
I did this when i changed all of my iMac core 2 Duo snow leopard data into the new mountain lion OS.
Everything worked fine
• how can i get the mountain lion backup when i upgrade it, so i can use this backup cd when my os gets crashed or do i need pay $19.XX again to get my mountain lion os?
You buy Mountain Lion from Apple. They keep a record of your purchase. If you need it again sometime later, there is no additional charge. You just download it again.
If however you want to save a copy yourself, that can be done. Here are the instructions: