Forget installing OSs like Windows on an external usb type or using an external casing through usb. The only external drives that are bootable are usb flash memory type drives. The main concept for external drives and casings is for storage/backup and transfers of data and files between systems. You take the drive not the entire system along with you.
well you can actually put linux on a externel drive. I have done it before.
however it is not easy, you need to change the kernel and what noe, it is easer to get a copy of pendrivelinux and put it on a flash drive, bt yes it is possable to use linux and use it on any computer