Adding additional block devices to XEN guests is accomplished using xm block-attach command. In this example I am exporting an external USB hard drive that was recognized as /dev/sda1 by the hosts kernel. Important thing to keep in mind is that you must use xvdX as the guest block device name.
This command will export /dev/sda1 on host machine to guest domain 6 as
/dev/xvda1 with write permissions.