Changing references to sda1 to sdb.

Based on tip in EC2 Docs:

http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/block-device-mapping-concepts.html

"On some devices, the names that you specify in your block device
mapping can conflict with the default block device names. To avoid this
issue, do not use names of the form /dev/sda[1-15]."