I have a pack of DVD+R DL and my computer spits them out when I try to burn one in toast. I am using a powerbook, and an external Lacie drive. Neither of the drives recognize the disks, and the only logical explanation is that my 4 year old neanderthal equipment predates this futuristic DVD DL disk, but that just doesn't make sense to me. It can play back this type of disk, but can't burn it. Can someone confirm, or better yet explain why this media is not compatible with this equipment?

There are DVD burners that can play Blu-ray, but not burn Blu-ray. Years ago there were CD burners that could read DVD, but not burn DVD. So it would not be unusual that there was once a DVD burner that could read DL, but not burn DL.

Open the Apple System Profiler and see what the make/model of the drive is. Then google it and you should be able to determine its capabilities.