Impossible to copy unprotected DVD with Disk Utility

Hi,

I'm trying to copy an unprotected DVD from a show with Disk Utility on El Capitan. I tried to create a disk image using the method File > New image > image from disk, CD/DVD Master.

The process launches et resolves but creates an image that weighs only 1.2 Mo.

If I burn this image (successfully) with Finder > Burn on disk, the disc then readsm when opend with DVD reader, "damaged parts" and quits.

What to I do wrong?

Thank you!