OSX – to restore the disk image, disk utility must scan it firs

Steps to create USB image:

1.Partitioning the USB drive.
2.Click the partition under the usb drive you want to restore the image on to. Then click File then Unmount.
3.Mount the .dmg image you want to restore first and use the mounted image.
4.Drag the mounted .dmg file to the Source
5.Drag the USB drive to the Destination

 الخلاصه : فك الملف  dmg اولا ثم استخدم اللي انت فكيته Install SED

وهتشتغل علي طول 

source: http://www.agady.com/osx-to-restore-the-disk-image-disk-utility-must-scan-it-first/