So I'm trying to clean install el capital on my friends MacBook Air but something weird is going on. Basically I've created a boot USB of el cap after installation when the computer finished I'm taken to the desktop which takes a long time to load and the USB is constantly breing read. The os/computer is super slow. Like mouse cursor studders. What's going on? I've clean installed before. My MacBook Pro and another macbook without these isssues. This seems as if the entire operating system is booting from a USB 2.0 or something. When I remove the USB and re start there is no operating system on the computer. Do something went wrong. I'm probably not doing something right. Any ideas??