External drives · Passwords and encryption
Can I encrypt just part of an external drive?
Short answer: Yes. On an APFS drive each volume can be encrypted separately, so you can keep one volume locked and others open, all sharing the free space. For a handful of private files, an encrypted disk image works like a password-protected folder.
Yes. On an APFS drive, each volume can be encrypted on its own. Add a volume in Disk Utility, choose an encrypted format for it (or Control-click it in Finder and choose Encrypt), and keep the other volumes unencrypted. All volumes share the drive's free space.
Another option for a few private files is an encrypted disk image: in Disk Utility choose File → New Image → Blank Image with encryption. It behaves like a password-protected folder you can keep anywhere.
See how to add a volume.