My friends Macbook Air died today from the start up disk being too full. I talked to an apple engineer in a shop and he said you can download OS X Mountain Lion being 4.4Gb, put it on a usb set up to reinstall. I downloaded mountain lion on my app store on my macbook pro. My friend carried on and erased the memory and had the USB plugged in. On the installation page you click continue to install. It then comes up to what drive you want it to install to and the USB stick is the only one that comes up. Anyone know where the inbuilt harddrive is and how to fix this issue? Any suggestions will be great.