Comment 37 for bug 666211

@David, from what you describe this sounds like something is blocking the device completely. And that could potentially be to a few reasons. However the call done by mdadm --examine should be a fairly basic read of some sector(s). That rather would tell that the one virtual disk is blocking. Which could be missing an interrupt or not having received one the first place.
Debugging those kind of things is very painful, as there is never much hard evidence. The problem usually happens earlier and then the process is being blocked messages get spitted out by anything that directly or indirectly is waiting on the IO. As I mentioned earlier there have been patches to address some interrupt issues. So it would be helpful to know what exact kernel version you are running. Also to confirm whether it also is a m1.xlarge or a different kind of instance. Another question would be whether you are bound to stick with Maverick or could try and move to Oneiric completely (or at least Natty).