How do I erase MacOS from HDD but keep personal files (i.e. convert a bootable disc to a storage disc)? I have a 2TB Seagate HDD which used to sit in my old Mac Pro. It has both the OS and my personal files on it. I want to sell my Mac but keep the drive and use it as an external HDD for my MacBook Pro. The drive has lots of movies on it and I would hate to lose them.
Folders currently on the HDD:

Applications
Library
opt
System
Users

Can I delete everything other than the Users folder?
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.