Clinton Stimpson authored Feb 23, 2017 and Brad King committed Feb 23, 2017

Adjust the image format to create the initial `dmg`, and convert only
when necessary.  This eliminates spurious

    hdiutil create -format UDRW

failures I'm seeing which may be related to an anti-virus scanner.
Running

    hdiutil create -format UDRW -verbose

indicates a "resource is busy" issue when unmounting an image.