On my 2017 27" iMac with Sierra 10.12.6, an mds directory is visible in the root directory. Why? On my 2009 13" MacBook Pro with High Sierra Beta 9, the vm directory is visible in the root directory. It has a currently unused swapfile in it. Why? Note that this latter machine used a patched installer to get High Sierra installed. Both machines work perfectly fine. Can either of these directories be safely deleted? If not, should they be re-hidden?