In the cloud everything can happen.
Actually everything will happen.
Therefore, in your design, you should be ready for failures: even if you expect your disk mounts to be there for you, they might not be. And you are doing auto scaling, it is most likely that one in a time they won't be there for you.
Therefore, to avoid hanging servers due to failure to mount disks and bad messages as the follow: "The disk drive for /tmp is not ready yet or not present", make sure your servers are not bound by your disks (otherwise you will not be able to contact your servers, or your OpsWorks will notify you that the server is booting forever).
Avoiding Waiting for Your Mount
The secret is a small option: nobootwait that will make sure your server is not waiting for the mount to be ready. You can configure it in your /etc/fstab, or even better in your Chef recipe:
mount "/tmp" do
action [:mount, :enable]
The right design will help you keep you system running in a cloud based environment