How to use QEMU with a live USB

Create a disk image for storage

We create a virtual hard drive named vm.img of size 20G.

qemu-img create -f raw vm.img 20G

The file format is raw, which has few features but offers performance.

Boot from a live USB

We will boot on a virtual machine with two hard drives, setting the first to the live USB. We use 2G of RAM memory and assume that the unmounted live USB lies in /dev/sdb

qemu-kvm -m 2G -drive file=/dev/sdb,format=raw,snapshot=on -drive file=vm.img,format=raw

This command may produce the error: Could not open '/dev/sdb': Permission denied. We need read permissions on /dev/sdb. There are several methods to deal with this, the simplest is to run the command as sudo, accepting the danger of running QEMU as root. Others are detailed in the blog post Accessing a restricted file on Linux and BSD.

Complete installation, shut down and then simply exit out of QEMU.

Boot from the virtual storage

Now you can boot from your disk file.

qemu-kvm -m 2G -drive file=vm.img,format=raw

If available, it is advisable to enable kvm by passing -enable-kvm to qemu-kvm. This will speed up emulation significantly.