Amazon doesn't put a filesystem on the device, you do.
> I don't know how Amazon (or the hypervisor) prevents access to the raw disk, where unallocated sectors might be found and scavenged even if the filesystem is erased. I guess they do something clever or we would have heard about people reading Zynga's customer database from a stale instance.
This is exactly what I'm talking about.
There are basically three approaches to doing this without the cooperation of the OS running on the instance (which you don't have)
1. the hypervisor zeros out the entire drive before the hardware is considered available again.
2. the hypervisor does encryption of the blocks with a random key for each instance, loose the key and reading the blocks just returns garbage
3. the hypervisor tracks what blocks have been written to and only returns valid data for those blocks.
I would guess #1 or #2, and after thinking about it for a while would not bet either way
#1 is simple, but it takes a while (unless the drive has direct support for trim and effectively implements #3 in the drive, SSDs may do this)
#2 is more expensive, but it allows the system to be re-used faster