Martin - actually one question in regards to this error. I have built the rump-nginx kernel. As I said it works fine on Xen.
Prior to the baked-in-root-filesystem, rump-nginx got its files from a couple of ISO images. Now that everything is baked in, those ISO image are not needed. But is it possible that on EC2 the kernel is still trying to find and mount those ISO9660 images? If so could this be causing the error? I had assumed that since it no longer needed to get files from the ISO9660 then it would just work but perhaps not? as
