It seems I'm running into some issues with MESS. I also tried Mini vMac and the floppy images from System 6 that do not work in MESS, do work in Mini vMac. I suppose the floppy images for both are the same?
The file that works in Mini vMac makes MESS print:
Device Floppy Disk [Sony] load failed: Invalid image
I tried it with the latest version from Subversion.
I also tried the pre-made System 6 images now, but while MESS starts, I get a floppy disk with a question mark in it. I tried a raw version of this image as well in Mini vMac, but it doesn't work either then.
Is it possible to get some more information from MESS when it doesn't like an image. I already found that when you specify a path to a file that doesn't actually exist for a floppy, MESS just starts. With a non-existing harddisk file, MESS does give an error though. This seems a bit inconsistent.