The number and size of ephemeral storage disks available for an instance depend on the instance type. For more information about which instance types support ephemeral storage, see Instance Types.
To activate an ephemeral storage disk for an instance, you need to define the disk through block device mapping when you launch the instance. For more information, see Defining Block Device Mappings.
You cannot redefine block device mappings after launch, or attach and detach ephemeral storage disks between instances. You need to specify all the ephemeral storage disks you want to activate when you launch the instance.
After the launch, you need to initialize ephemeral volumes from the ephemeral storage disks to use them. For more information, see Initializing a Volume from an Instance.
To avoid conflicts with device names when restarting your instance, we highly recommend naming your ephemeral disks before naming your volumes. You can also start naming your ephemeral disks from /dev/xvdb, and your volumes up from /dev/xvdz.