Add/remove disk to Xen instance

To add a physical disk to a xen instance..

xm block-attach ID phy:/dev/sda1 /dev/xvda1 w

Replace "ID" with whatever instance id you need (get from xm list)

To see what block devices are attached..

xm block-list ID

To remove one..

xm block-detach ID Vdev

Replace ID & Vdev accordingly.