Wow... just got home to see the read-only image I started this am, finished half an hour ago....
383GB hard drive image - took 3hr 50min to create the sparse-image, then another 5hr 20min after this to complete.... glad I was at work!
It mounts in a second or two and is much faster to navigate than sparsebundles I have, then ejects in a split second too - is this normal??
I certainly won't use these for anything other than a record once, leave-as-is-archive type backup thats for sure (at this size anyway)