removes automatic eject on OSx. This only worked sometimes because it depends on how long OSx takes to remount. Therefore it is simpler to instruct the customer to do this.
Adds error checking. In particular, if we fail to unmount a disk, which likely means we are about to overwrite something important. Also we examine the result codes from dd and gzip.
@alex5imon @pazdera @tombettany This is the branch which Ricardo has tested, it would be useful if I could merge it. I will be basing any work I do off this one.
This PR: