RTFM: encontrei minha resposta nos documentos de armazenamento de instâncias do Amazon EC2 .
Instances that use Amazon EBS for the root device do not, by default, have instance store available at boot time. Also, you can't attach instance store volumes after you've launched an instance. Therefore, if you want your Amazon EBS-backed instance to use instance store volumes, you must specify them using a block device mapping when you create your AMI or launch your instance. Examples of block device mapping entries are: /dev/sdb=ephemeral0 and /dev/sdc=ephemeral1.
Portanto, no meu caso, como escolho um dispositivo raiz do EBS, preciso recriar a instância com o volume do armazenamento de instâncias.