Hope I got this straight, and this is the place to post questions:

Have made a persistent USB of Ubuntu 18.04. Which worked fine until I tried to boot my mac mini (2009) It booted fine for a couple of times then the boot partition got messed up. It won't even boot on a PC now. Here's the error message:

(initramfs) Mount: mounting/cow on/root failed: Invalid argument overlay mount failed.

I put a lot of time into creating and setting this up with apps etc. Is there a way to rectify this? 

Thanks, Charles