Hit this strange problem today on RHEL5.6. Saw a QEMU disk image (qcow2 format; let’s call it “image2”) which was a copy-on-write link to a base OS image (let’s call it “image1”). Trying to start a KVM virtual machine using it resulted in the error “Could not open disk image”. After checking various things including that both images were OK (“qemu-img” check, loop mount) permissions, SELinux etc., it turned out that what this error actually means (in this case) is “You are using a copy-on-write image with a backing file of format ‘vpc’. This doesn’t work”. So, “qemu-img convert” solved the problem.
Good example of some poor error reporting…maybe it will help someone else out.