Problem sharing virtual floppies between Minix2 and Minix3

I want to share a virtual floppy between Minix 2 and Minix 3, and I
can't seem to get it done. I must be making some really silly mistake,
so I'll describe my whole setup: I'm not sure exactly where my
sillyness is located.

I have both Minix 2 and Minix 3 running under Q on an Intel Mac. (Q is
basically Qemu with Mac GUI stuff). I want to use a virtual floppy to
transfer files from Minix3 to Minix2. (Why? Because I have virtual
networking running under Minix3, so I can ssh out to the Mac itself to
transfer files, and the Minix2 I use has no working network on my
setup. I also don't want to mess with the networking stuff on Minix2
because I need it for a network assignment:-).

So, I created a virtual floppy, formatted and mkfs'ed it on the Minix3
VM, mounted it, copied the files, unmounted, ejected it, attached it to
the Minix2 VM. Mount fails: it doesn't recognize the filesystem.

Ok, try the other way around: recreate the floppy on the Minix2 VM, go
through the whole process, try to mount on Minix3: not recognized as a
filesystem. Redid the whole rigamarole with a type 1 filesystem, both
ways: no go.

Tried various times to fsck the filesystem created by one Minix on the
other, but at best it says "bad superblock".

If anyone can point out my mistake or give me pointers for further
research that would be much appreciated,

Jack