Erasing Mac HD and keeping Parallels If I erase my Sierra OS startup disk, will it erase the Parallels partition too? I can't seem to find an answer anywhere.
If by Parallels partition you mean:


the Parallels Virtual Machine (or .pvm file) usually located within the Documents > Parallels folder, then yes this will be deleted.
your running Parallels from a Bootcamp partition, then as long as you're not erasing the Bootcamp partition (i.e. you're only erasing the macOS partition and not the entire drive), then no this will not be deleted.
something else, then please clarify your question.