I partitioned my mac to have ML. The size on mac appstore says 1.5GB. I have 3GB on my ML disk. It says i need more room. How much does Xcode 4.5 need? When making the partition I have 11Gb partition for Mountain Lion. But it said in disk utility i had 5GB remaining. I hadn't even used it yet so what is that 6GB? After install i had 3GB as mentioned above. Is it wise to install ML on an external HD that is used for backups?