How do I take a pre-formatted software installer thumb drive and make a disk image for storage on a backup external hard disk? I just bought a mail-order USB 3.0 thumb drive (all the thumbs for sale around here were USB 2.0) that I intend to use for bootable software installations. One of my first projects will be to use the USB 3 thumb to clean-install MacOS Sierra. I am following instructions from this MacWorld magazine article. I want to be able to erase the USB 3 thumb and re-use it for other installation purposes. If I want to clean-install Sierra on another Mac in the future (a family member's MacBook), I have to keep the installer disk image in safekeeping on an external hard drive so I can retrieve it later. So I have to find out how to make a disk image of the installer that is now on the USB 3 thumb for safekeeping now, and to be able to retrieve it and reformat the thumb at a later date. Any suggestions on how to do this are welcome.