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I would like to summarize how to create ISO files of CD/DVD media or directories.

Creating an ISO image of a CD/DVD disc

First, make sure that your disc is currently *not* mounted.

$ sudo umount /dev/cdrom

Then, create an ISO image disc.iso of the disc by dd command:

$ dd if=/dev/cdrom of=disc.iso bs=1024

Creating an ISO image of a directory

To convert a directory dir into an ISO image dir.iso, use mkisofs command as follows:

$ mkisofs -r -o dir.iso dir