Customer instances have no access to raw disk devices, but instead are presented with virtualized disks. The AWS proprietary disk virtualization layer automatically resets every block of storage used by the customer, so that one customer’s data are never unintentionally exposed to another. AWS recommends customers further protect their data using appropriate means. One common solution is to run an encrypted file system on top of the virtualized disk device.

[1] http://awsmedia.s3.amazonaws.com/pdf/AWS_Security_Whitepaper...