Ok, this one has really been bugging me. I've been using Apple Compressor because it is just way (like 10x faster) than Final Cut Pro > File > Share > Apple 4K. But, every so often I get this annoying RequestCVPixelBufferFrame (how user-friendly message).
There is a lot on the Internet (and no, you shouldn't use Reddit or Twitter anymore!), but there is a lot posted about (which I don't have)
Some folks say there is a bad frame in there, but I find that a little hard to believe since the input was all digital, and then others say that it is because your external drive wasn't formatted as APFS. In this case, I've gotten the error on the internal drive and on an external drive.
But I got a bit of a clue when I tried to export from Final Cut Pro and it said, you can't export this as you only have a Proxy Media, you need to also create Optimized Media. Turns out that because I was running out of disk (Final Cut Pro in 4Kp60 mode eats up a lot of disks), I had turned off Create optimized media on import. You do this in Final Cut Pro > Settings > Import > Transcode and you can click on Create optimized media which is easy to export or you can click on Create proxy media which FCP then uses to be much faster.
You can also check to see what you have by going to a clip, right-clicking on it, and looking at Transcode Media, what is greyed out is what you already have. Once I enable Optimized Media, then magically the File Send to Compressor works fine. Now this is going to be very fat on the hard disk, so when you are done, you might want to clean it all up. What is happening is that most media is very compressed which makes it very slow to work with, so instead, you create something in a loose, fat, and very easy-to-edit format, in this case, ProRes422.
Conjecture: Only creating Proxy is a problem
I'm not really sure but as soon as I turned on Optimized Media, the problem went away. I don't really understand why, but if I don't have Optimized Media, I can't send to Apple Compressor, so I'm guessing the Compressor can't deal with the original files (typically AV1 or some other compressed format) and it gets sent the Proxy Media files which are too small. That is what the File Export natively basically says. So the net is I'm turning on Optimized media from now on. The problem is that this is a very fat format so it chews up hard disks like crazy
Why do you need Proxy Media? Mainly you don't
If you have a really slow computer, you can use Proxy Media so you get three copies of all the data, the original clips, the optimized fast very, and the proxy version. On an M1 MacBook, you are probably going to be fine with just the optimized version.
All these things are called render files, so when you are done with things, you can get rid of them. But, then when you want to edit again, you probably want to go to the clips and recreate them. Note that the Settings is just for what happens on Import. As an aside, you can see how much space is taken, but clicking on the Project at the top and then you will see in the right pane, at the bottom "Storage Used for Media and Motion Content". In my case, 20 podcasts are using 80GB origin, 517GB optimized, and 78GB Proxy
Getting rid of Optimized and Proxy when done
There are two ways to clean things up, at the top Project level, you can go to File > Delete Generated Project Files. But it's more likely, you will go from Event to Event (each Event for me is a show I'm making and then you can select File > Delete Generate Event Files. And then you can select what you want to delete, the best candidates are these Optimized Media Files and Proxy Media as they are huge.
Note that as you go through each event and you choose to delete, if you look at the property pane on the right, you will see Available Media Representations and a green light means you have them.