[dnanian]

Nope -- I think you're probably not using the "Disk image..." choice in the destination pop-up, but rather mounting them the 'old way'.

So -- choose "disk image..." in the pop-up, make sure it's set to "Read/Write "Sparse" image", and then pick the image file. Ignore the warning about overwriting -- it's just going to update that, not overwrite.

That's all you need to do.