If it is automatically mounted, you will need to unmount it. On my laptop, my dvd drive is /dev/sr0.

To figure out what device it is, you can always mount the device or if it is already mounted, type
cat /etc/mtab | grep udf.

To create the iso/disk image, the device must not be mounted. To unmount your DVD drive/CDROM drive;
#DVD Drive
sudo umount /dev/sr0
#CDRom Drive
sudo umount /dev/cdrom

Now, we will copy the raw data from the dvd/cd to a disk image using the dd command. The dd command stands for “dataset definition” but I like to personally remember it by “data dump”.

Be VERY careful using dd, one mistake can make a hard drive completely unusable. Caution is advised. Lot’s of useful information on dd can be found on the wikipedia.

Now for the command;
dd if=/dev/sr0 of=/home/user/diskimage.iso

This will copy the raw data from /dev/sr0 (DVD drive) to the file /home/user/diskimage.iso