Perhaps it may help others,
but I found that my solution to this problem was not a bad rip / iso, but instead a faulty USB stick.
It turns out that despite it seeming to contain 64 GB, its real capacity was only 8 GB.
When I exceeded this real capacity, then the "reading disc" message would appear, likely due to it trying to access data that's not really there.
I found this out by hours of trial-and-error, noticing that all games after that initial 8 GB point no longer load from the USB.
This may be an unfortunate consequence of using cheap USB drives.