The copy of Returnil sandboxing software that I installed from here -- presumably about a year ago -- just expired. Wanting to replace it with the free version, I downloaded that version off the website; when I went to install the free version, it said I had to uninstall the previous. I did that.
When the GAOTD version of the software was uninstalled, it took with it the "Z-drive" which Returnil had created to store everything I'd downloaded while in protected, sandbox mode. All that stuff is gone.
I had a vague hope that installing the new version, and there-by re-creating the Z-drive, would put it and its contents back into the directory structure, but of course that didn't happen.
So, um, warning, y'all: move your Z-drive stuff onto a partition you have control of before uninstalling Returnil.