Hi Nicholas-Campbell
Thank you for bringing this to my attention. It's a different problem that you've found. The issue in the screenshots was due to swapping the disks during game play. The game continues to read the disk as if it still has the game file content on it. The disk swap has removed the game file so it isn't there anymore. This swap can't be reliably detected.
Please be aware that save game sizes also vary depending on game complexity, as well as how far into the game you are. In general, the interpreter has been successfully detecting and reporting that the disk is full, and gracefully returning. It turns out that the exact current size of the game, and the amount of storage required for this game to save progress hit an edge case where saving the last fraction of a sector wasn't handling the error as well as the rest of the save function does.
Had the game been slightly larger, it would have worked correctly and this would have gone unnoticed. An update to the code is being tested and will be available soon.