How do I erase MacOS from HDD but keep personal files (i.e. convert a bootable disc to a storage disc)?
Sure you can.
Once it's no longer actually being used as your boot drive you can treat it just like any other data drive. It will let you delete anything you want - just make sure you keep the good stuff ;)
I doubt you will even get any permissions issues at all, but if you do, you can Get Info on the drive itself &amp; check the box at the bottom to ignore ownership.