Nobody is going to find your 'missing' 30 minutes of screen recording. If your video fails to SAVE, whether its a Cocoa error or something else, your 'recorded' video is gone. Again, if the source of whatever you're recording is protected or copyrighted your screen recording will fail to save and you'll likely get the Cocoa -1 error message, which will only tell you that your screen recording didn't save but the error code will not explain what in particular caused the error.
An example is when I want to show somebody a screen recording of how to change a setting or explain a process with a screen recorded video. It saves correctly every time. But if were to try to get a screen recording of a movie or a copyrighted clip, at the end of my recording I'll get the Cocoa -1 error. Not Apple's fault - their device can't allow you record just anything you want - if you could do that you'd never have to pay for a movie again, you could just screen record it and share it freely.
I'm not going to pretend to know how particular devices running certain iOS versions behave when it comes to screen recording. I've read how restarting your device can help, or checking certain settings like power saving mode, or uninstalling and reinstalling apps will help, or clearing internet history and website data in hopes that it will make the error go away. Maybe that will help in some instances but what it comes down to is that iOS realizes there is a problem with your screen recording itself or the storing of the recording and that contributes to your Cocoa -1 error.
