An encrypted disk image is a file that opens as a volume after you unlock it. It can hold a small set of private documents, but its protection depends on password handling and closing the unlocked volume when you finish.

Create and test an empty container

In Disk Utility, choose File, New Image, Blank Image. Follow the secure-image instructions for your macOS version, selecting a suitable size, encryption and a writable image format if you need to edit its contents. Use a clear filename that does not itself disclose sensitive information.

Create a strong password and store it securely somewhere you can access independently of the image. Apple warns that forgetting the password prevents access to its contents. Do not make the encrypted image the only place where you record its own password.

Before adding valuable documents, put a harmless test file inside. Eject the mounted image, reopen the image file and confirm that the intended password unlocks it and the test file is readable.

Understand what remains exposed

Copy the required documents into the mounted volume and verify them. The originals outside the image remain separate copies; creating the container does not automatically encrypt or remove those originals, application exports or other existing backups.

Apple explicitly advises ejecting the image after use. While it remains mounted, someone with access to the active computer session can use its documents. Closing a document window alone is not the same as closing the protected container.

Keep an independent backup of the image and test access to that backup before relying on it. Encryption protects confidentiality, but corruption, accidental deletion or a lost password can still prevent recovery.

This workflow is intended for documents on healthy storage. Do not create an image of a failing disk as an improvised recovery experiment; Apple's guide cautions that such an image may not be a reliable backup.

Source: Apple's secure disk-image guide.

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