I have just spent the last 2 days trying to figure out a way to rip the content of my DVD and save it into the ISO image file format (a file with a .ISO extension) so that I could mount this file as a 'DVD' under Oracle VM Virtualbox. I thought this would be an easy process but it did not turn out to be so.
After various attempts, I ended up using a freeware by MiniDVDSoft called Free DVD ISO Maker that allows me to easily extract directly from the DVD and save the content into a file in the .ISO format. Virtualbox mounted the .ISO file successfully, just like a real physical DVD. Wonderful little software. And it works beautifully without any problems.
Below is a description of my other (failed) attempts. It is just for my reference and also to others who may be interested.