This looks like a problem with the block mapping when the AMI is registered.
After firing up an t1-miro instance, the Metadata shows: block-device-mapping: ami: /dev/sda1 ephemeral0: /dev/sda2 root: /dev/sda1
While one that does have an ephemeral mapping shows: block-device-mapping: ami: /dev/sda1 root: /dev/sda1
This looks like a problem with the block mapping when the AMI is registered.
After firing up an t1-miro instance, the Metadata shows: mapping:
ephemeral0: /dev/sda2
block-device-
ami: /dev/sda1
root: /dev/sda1
While one that does have an ephemeral mapping shows: mapping:
block-device-
ami: /dev/sda1
root: /dev/sda1