To make an ISO from your CD/DVD, place the media in your drive but do not mount it. If it automounts, unmount it.

ISO from DVD drive

dd if=/dev/dvd of=dvd.iso

ISO from CD drive

dd if=/dev/cdrom of=cd.iso

ISO from SCSI drive

dd if=/dev/scd0 of=cd.iso

To make an ISO from files on your hard drive, create a directory which holds the files you want. Then use the mkisofs command.

mkisofs -o /tmp/cd.iso /tmp/directory/

This results in a file called cd.iso in folder /tmp which contains all the files and directories in /tmp/directory/.