I'm thinking that the OS-level diskcopy tool doesn't expect to see such a wacky layout, and would just fail to track along what it expects to be concentric circ

I'm thinking that the OS-level diskcopy tool doesn't expect to see such a wacky layout, and would just fail to track along what it expects to be concentric circles. Only the software that's aware of the custom layout would be able to read the disk, by taking direct low level control of it. And if the data is laid out in a true spiral, mimicking that with 1/4 shifts every 90 deg. might be close enough to never lose track of the, erm, track.