a copy of a sparsebundle doesn't work?

[camner]

As I rebuild my system, I want to make "snapshots" of the volume as I go along. What I thought I would do is to make a sparsebundle (say, called "bu_1.sparsebundle") then make a copy of that sparsebundle using the Finder, rename the resulting file (say to "bu_2.sparsebundle") and then use SD! with Smart Update using "bu_2.sparsebundle" as the target. This doesn't work. SS fails with a "can't set permissions" error. Is there a reason this won't work?

Also, SD describes a sparsebundle as the most efficient of images, yet making a Read only .dmg with high compression results in a far smaller image than making a sparsebundle. Why is that?

Thanks, as always.