Thanks, everyone!
For now this will be provisioning physical drive for a box -- and "everything" will be there for now. So OS on one drive, and DB on another.
I've run into programs before (mostly on Mac/Win) that are exceedingly not happy if they're run on a drive other than the OS.
Since many people partition data and services under pg, I figured it would be okay -- but I couldn't find anything in the docs and wanted to check.