Memorex DVD+R Dual Layer & Power Mac G5

Yes, older Macs don't support dual layer.

Go to Apple menu -> About This Mac -> More info -> under Hardware, select Disc Burning. Unless it says DVD+R DL, it won't support dual layer burning, and presumably reading as well. The specs of what it can read are in the Apple Spec database as well for the most port:

http://www.info.apple.com/support/applespec.html

If you can tell on the ATA bus, what DVD drive model you have, visit the manufacturer's website for more info on the drive support if your Mac model isn't listed above.

Yes, they are relatively new. For more on the history of DVD, see this page:

http://www.videohelp.com/dvd

I typically buy double sided DVD-R disks to burn more than 4.7 GB of data. These you flip over when you want more content than can fit on one side of a DVD. Alas you have to split up the content. Depending on the type of content this can be done by segmenting an image, or compressing the data itself and segmenting a compressed file. What's odd is that some disks which appear on the surface to be dual layer because of their single sided capacity, such as the Tiger installation DVD, are in fact readable by all DVD drives that are on Macs, whereas the commercial DVD+R DL is not. I don't know the difference between commercial and consumer grade Dual Layer disks, but there does appear to be one which makes them readable on Commercial grade whereas Consumer is not on older DVD players.

Only the newer drives can burn dual-layer DVDs but all DVD drives should be able to read at least some dual-layer disks. Commercial movie DVDs have been dual-layer for years. Recordable dual-layer is new and older drives may not read all brands. Try Verbatim DVD+R DL disks. Memorex has a bad reputation.

I've used Verbatim Dual Layer disks, and they aren't readable in my iMac G5 either. Neither are they readable in my Samsung stand alone DVD Player after burning them as a movie DVD with Toast using my Pioneer 108 based external Superdrive manufactured by OtherWorld Computing. And my mother has one stand alone DVD player that won't play them either. Consumer grade DVD+R DL is only supported by the newest players.