I looked that up before, apparently its a common warning and doesn't actually affect the writing process. cdrecord automatically corrects for it

Yeah I've had that warning for every disk I've ever burnt.

Are the burnt CDs viewable in other machines/OS's? The cdrecord output looks fine to me. I'm wondering if it's just a mounting problem rather than a burning problem. If you try to mount a CD manually as root does 'dmesg | tail' give any useful info/errors?

Doesn't have anything to do with the media.

I love sites like the one allend quoted - that state to avoid CMC media, and to use Verbatim. Hmmmm, wonder who makes Verbatim.
Verbatim is currently made in two factories. One by MBI in India, the other by CMC in Taiwan. Verbatim did at one time make their own media, but once they re-evaluated their price and procedures, found that CMC could make the same (if not higher) quality media for less money. Verbatim licensed their MCC MID plate to CMC. Just check the the made in label on your media. If it's made in Taiwan, it's CMC. Made in Japan its Taiyo Yuden. Made in Singapore, hold on to these, their old and actually made by Verbatim

Of course CMC does provide a variety of media grades to their customers. Even the lowest quality meets or exceeds standards. The common problems come from non-optimized write routines in firmware, which why OPC strategies went into place. But in reality, if you want something great, stick with Yudens.

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mount /dev/sr0 /mnt/tmp <--- to mount the first optical drive to /mnt/tmp

What happens when you chmod -x the Virtual Box rc scripts and reboot?
Considering Virtual Box has it's hooks into your optical drive (It allows pass through operations), perhaps there's something holding onto the drive. Also, while you are rebooting, throw in that Slackware DVD (or another disc) to verify the drive can at least properly read the media, without operating system interference.

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OK - Point taken about the error. My apologies to all.

My personal experience with a recent spindle of CMC disks is that I got some bad burns. Lowering the write speed did help.

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Nope. I have Windows 7 on Virtualbox (a few programs I specifically need won't run on wine), and after each change I've tried to open it in that. The error it gives is that the disk may be corrupt, or something like that (don't remember the specific wording).

And in a new development, I found my USB-powered DVDRW drive and plugged it in to try to burn a disc (same spindle, same image) and, miraculously, I can burn properly with it.
I know I've used my current drive in the past because I installed Linux on this computer with the same drive. So unless my drive is just going bad, the only thing I can think of that might account for everything is that there's some drive order problem buried in a config file somewhere, where the first drive detected is unusable outside of a virtual machine while subsequent drives are fine. Not to mention the fact that blank discs are recognised by the writing software.

So my problem is halfway solved anyway. I have burning capability, just not with the drive I want.