Why is my iMac no longer recognizing Memorex CD-R and DVD-R media that it burned previously?

Question

So this has happened before to me with other computers and other drives, I've had burners refuse to recognize some brands of blank discs before, but I've never really figured out the issue or had a drive start to refuse to recognize a specific brand after a few years of previously recognizing and burning them just fine. Here's the problem:
I have an older (2007) Intel iMac with the standard superdrive they shipped with. It wasn't particularly picky in the past with accepting different brands of blank CD and DVD media, though I remember that it did refuse a couple brands but I don't remember any more what they were. It did however always accept Memorex brand CD-Rs and DVD-Rs, and I literally burned several hundred DVDs and CDs on the internal superdive. All of a sudden, today I go to pop in one of the CDs and it tries to read it for about 20-30 seconds and then spits it out, no message, just rejects it. And the same thing happens with the DVDs. It's just the Memorex brand ones, older Sony and Verbatim CDs and DVDs are still reading just fine. I tried a blank Memorex CD from the same spindle that it previously burned discs from and it does the same with the blanks, blank DVDs from the same DVD spindle as well, spits them out when running Toast or Disk Utility or iTunes and trying to burn to them.
So it apparently just decided that it doesn't like these Memorex brand discs, but they worked fine for the last 5 years. This is a pretty big problem as like I said, I have hundreds of CDs and DVDs with backed up data. They aren't dirty or scratched, they are pristine. Commercial CD's and DVD's read fine, and other brands of blanks, both already burned and blank, read fine as well. It's literally just the Memorex that this same superdive burned over the years.
In the past, I had a couple extra DVD burners laying around and some external cases and had the capability to use a different drive if my internal wasn't playing nice with a brand, but I liquidated all my extra hardware a while back and don't have any other readers or writers, internal or external, that I can hook up to see what happens, unfortunately. I'm thinking that with one of the more recent OS updates, somehow support for Memorex brand discs was tossed out so the computer won't recognize them anymore. I don't know why else it would change what brands it supported like this on me. And I'm also guessing my only solution to be able to read all my hundreds of backed up discs is to buy another DVD drive and hook it up and hope it plays nice with all my discs. But that's really frustrating and I'd love it if somehow it would just recognize all the discs it used to without having to spend any money.
And I'm hoping beyond hope that this is not a unique problem I have and others have the same issue with Memorex discs and someone figured out how to get them recognized again, but I'm not holding my breath.
Any advice or musing on this issue is much appreciated. Thank you for taking the time to read about my problem and hopefully to offer a solution!

Answer 1

Thank you very much for your response, Klaus1. Yeah, when I was searching the forums for an answer to this problem before finally posting, I gathered that Memorex blank media isn't recommended and of poor quality, unfortunately, which is of course too late to help me with my current problem, but knowing that now I'll make sure I don't continue to use them in the future. I used them for many years, as long ago when DVD burners were first available I had a very hard time with Toast successfully finishing a DVD-R on an internal Panasonic 2x burner on my G3 tower, and for some reason I had good luck with them (Memorex). And then I had a lot of them on hand and they've stood up well over time (not flaking, etc.) so I kept using them over a few Mac upgrades. It also had to do with DVD players we had being compatible and able to read them.
Anyway, I'm rambling, point being I used them for lots of various reasons, but I should have kept informed in the more recent past and learned the fact that they're a poor choice and switched, my own fault. I just don't want to look totally stupid, like as if I had chosen them originally for no reason.
I went ahead and reset the NVRAM and also reset the SMC, and I had tried my cleaner disc with little brushes earlier today for the heck of it, too, but no luck still. The drive just changed it's mind and no longer likes the Memorex discs, apparently end of story and no changing it, sigh. It still really baffles me that it had no problem with them for nearly 6 years up until a few months ago, the last time I used one. So weird.
And no, I don't have the data on the discs stored anywhere else, so it's access them or nothing. I rummaged around and found an external 5.25" firewire case, and I do have a PC (yuck) I can borrow the internal dvd drive from and toss in the case and hook up to my iMac to see if it will read the discs and let me directly re-burn them to better media. I used to be very knowledgable about hardware and Macs in general, but I admit I haven't kept up my knowledge, and I don't know whether that will work - if my iMac is spitting out the discs because the drive doesn't like them and therefore a different drive might work, or if it's spitting them out because the system doesn't like them and switching drives will do nothing for me?
If not then I'll have to resort to finding another Mac I can hijack for a long time to re-burn all these discs I need, as obviously a machine with one drive is going to take a long time - putting in a disc, copying it to a local drive, ejecting it, putting in a new blank, and re-burning the data about 250 times. And a lot of baby sitting the machine as I don't have a robot to keep doing it for me. Sigh.
Thank you again for your advice, I greatly appreciate it even though I still struck out.