You can either attach an EBS Volume to an running instance or configure it to be attached to an instance (at boot time) when it is launched.
When attaching a volume, you must specify the following information:
Volume to Server Attachment
Server - Select the server to attach the EBS Volume to.
Device - Select the mounting device on the server instance to attach the volume to. Since the server instance has not been launched, you must select the appropriate device mount depending on which operating system the server instance will use; otherwise, you will be unable to attach the volume. Choose an "/dev/sd" device name for Linux instances or an "xvd" device name for Microsoft Windows instances. Several devices have been predefined for you; however, if you require more, you can select the "Override" option to specify additional device mounts using the Xen Virtual Drive notation (e.g. /dev/sdn or xvdi).
Attachment Options - If the selected server is running, you can either attach the volume immediately (Attach now) or the next time the server is launched (Attach on boot).