Apple did a massive overhaul of Disk Utility in El Cap and it appears that burning DVD images from within Disk Utility is no longer supported. I guess from Apple's perspective, optical is increasingly dead technology. Sometimes, though, out here in the real world, we have to suck it up and use downrev media (if for no other reason than to accommodate narrow-minded developers like the folks who wrote the installer in CentOS 6.7, which can't find its install media when you convert the image and boot from a thumb drive).
Ahem. I digress. There are various how-to notes out there telling you that you can burn an iso image directly from the finder but that didn't work for me and I don't know why.
But there's a command line tool for that. The incantation is:
hdiutil burn CentOS-6.7-x86_64-bin-DVD1.iso
and bingo, my Buffalo external bluray writer with a DVD+R blank in it sprang to life.