My eMac will not read a DVD-R dual-layer disc that I've just received. It spins the disc, has a good hard think about it, and then spits the disc back out.
Am I finally falling out of date? Is this because I have a (relatively) old Mac (built between 31/05/2004 and 06/06/2004, according to CoconutID), or is there something else I should be considering? A firmware issue, perhaps?
Thank you.

It's the Dual Layer that is the problem, i would think. It seems, like my iMac, that your eMac can only read DVD-R Single Layer discs.

EDIT: I could be wrong, though, as it just hit me that DVD movies are usually dual-layer, and my Superdrive can play them just fine.

Perhaps you could try a different DL disc?

If it is a DVD+R Double Layer spec disc, then it may not work as tommelbourne has stated. DVD+ formats are not officially supported by the DVD board, whereas DVD- formats are (hence why DVD- is used for Video and DVD+ are avoided like plague by small time video guys who can't afford stamping).
That is most likely your issue, unless of course you mean it wont play a store bought factory stamped dual layer disc. Then your drive needs to be replaced methinks :/

Age of the computer isn't the problem, Granny, the culprit here is most likely the disc you were sent. My G4 is nearly eight years old, the DVD burner in it four years old, and the only time it has ever had problems reading a disc was trying to read something burned from a failing (brand new Pioneer) drive.

Not all media is created equal, and the same applies to a lot of the burners on the market these days. The most probable cause is that the disc was a cheap-as-chips brand disc, burnt on a PC, in a crappy Pioneer burner, at full speed, and without verification being done afterwards.

Get whomever sent you the disc to send another copy, but ask they burn slower, and/or use different media, and/or do a verify after the burn.

If the DVD-DL you were sent is a movie, try it in a normal DVD player. If you have access to another computer (Mac or Windows) with a modern-ish DVD reader, try the disc there ... if it works, then you could have a minor alignment issue with the DVD drive inside your eMac, so that will need a clean with a CD/DVD lens cleaner disc, or at the very worst, total replacement.

Don't discount your eMac (or it's built-in drive) until you have tried other options.


Brains

Thanks Brains.
The disc in question was burned on a G5 iMac ... not sure what speed, but I'll do as you suggest. Since the disc also won't mount on my iBook, I am likewise coming to the conclusion the disc is at fault.

Disc is at fault!

My eMac ATI can play back DVD movies and DL-DVDs - your drive doesn't need special support for DL-DVD reading because it's a part of the format, DL-Writing on the otherhand is different because single layer was only really available for writable media when the standard was established.