PowerISO 4.7 Issue

I have an image of MAC OS X Leopard as a dual layer DVD.
Presently, the image has a .dmg extension.

I cannot use ImgBurn or Nero to read this image.
I can use PowerISO to do so, however the image is read
as a Hybrid MAC/PC Image of 510MB only.

Now, the actual size of the image is nearer 7GB.
I have dual layer media.
These are Verbatim, DVD+R DL
I have checked my DVD burner supports these - it does.

When I change the extension of the image to .iso
PowerISO will read and display a MAC HFS+ image of 7128MB.

When the extension of the image is .iso, I still cannot use
ImgBurn or Nero to access this image.

ImgBurn is
Nero is Enterprise Edition

I would like to make a couple of backup copies of the image
Is this a problem where the image has multi sessions?

How can I easily make a back up copy

Thanks in advance