When creation of an EBS snapshot is initiated, but not completed, the EBS volume:
Can be used while the snapshot is in progress.
Cannot be detached or attached to an EC2 instance until the snapshot completes.
Can be used in read-only mode while the snapshot is in progress.
Cannot be used until the snapshot completes.
Snapshots occur asynchronously; the point-in-time snapshot is created immediately, but the status of the
snapshot is pending until the snapshot is complete (when all of the modified blocks have been transferred to
Amazon S3), which can take several hours for large initial snapshots or subsequent snapshots where many
blocks have changed. http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ebs-creating-snapshot.html