i extracted a ubuntu 12.04 amd64 in usb by startup disk creator and the original image is deleted when i install ubuntu 12.04 from usb now how can i remake iso image from usb without re download iso?
Just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command below.
Again it is EXTREMELY IMPORTANT that you make sure you're using the correct device name before executing the command above. Otherwise you run the risk of clobbering the data on some other disk device that may be critical to your system.
Source: Ubuntu Forums