Why won't my internal MATSHITA DVD-RAM LF-D211A mount a blank disc or burn one?

The System Profiler confirms that it can read CDs and DVDs only. Has it ever had a burn capability? If so, it has now failed and your best course of action is to buy an external DVD burner.

It seems it really is an unusual, very old drive in that it is a DVD-RAM drive, not DVD-ROM. From a general web search it appears is likely does not have burn capability, or at least the dozen posts I read did not result in it burning a ROM disc.

https://www.macgurus.com/forums/archive/index.php/t-22102.html

I had that machine (PowerMac G4, DP500, Gigabit Ethernet). Expensive and bleeding edge at the time, but the optical drive proved to be of very limited utility. The DVD-RAM will burn only Type 1 and Type 2 DVD-RAM discs. It should also mount and give read-access to Type 1 and Type 2 DVD-RAM, DVD-ROM (including DVD-Video), DVD-R that you've burned on another machine, and CD discs. Here's a long-ago archived info sheet on DVD-RAM technology: http://support.apple.com/kb/ht3072.

When prices for optical drives came down, I replaced the DVD-RAM drive with a more conventional CD/DVD burner.

Here's a long-ago archived info sheet on DVD-RAM technology: http://support.apple.com/kb/ht3072 .

Hey, it says, "Last Modified: Jul 19, 2013" That's not that long ago! 😝 Actually for Apple it's pretty good given that generally anything 4 years or older is well on the road to being disowned, and that document held out for over a decade probably. In fact, it's amazing it is still there!

You're right. I didn't notice the "last modified" date at the bottom.

DVD-RAM technology never really caught on--sort of like Apple's "hockey puck" mouse from the same era.