When a Mac project needs a stable reference beside editable working files, create a read-only disk image from a dedicated reference folder and save the image next to the project. Keep the live project files outside it. This gives you a copy that opens for viewing while preventing edits to the image’s contents through normal use. Disk Utility can create a read-only image from a folder; Finder’s Locked checkbox is quicker for one file, but it is an accidental-change safeguard rather than a read-only container.

Choose the right kind of read-only copy

Use a read-only disk image when the reference contains several files, such as a brief, approved design exports, source PDFs, or a delivery specification. It packages a folder into one image file that you can mount in Finder for browsing. Apple describes the read-only image format as one that cannot be changed after creation. A read-only compressed image is also an option when storage space matters. Apple’s Disk Utility guide

For a single PDF or image, you can instead lock the document from Finder: select it, choose File > Get Info, expand General, and select Locked. Apple says a locked document warns when you move it to the Trash, but it can still be deleted when the Trash is emptied. In Preview, a locked PDF or image can be edited by its owner after unlocking; other people can make changes by duplicating it. Treat this as a guard against routine accidental edits, not as an immutable archive. Lock a document on Mac · Lock PDFs and images in Preview

Create a read-only image from the reference folder

First, make a folder containing only the files you want frozen in time. Put it alongside the editable project folder, for example: Project/Working Files/ and Project/Reference/. Check that the folder includes what you will need later; the image records a copy, so later changes to the originals do not automatically update it.

Open Disk Utility from Applications > Utilities. Choose File > New Image > Image from Folder, select the reference folder, and click Open. Choose where to save the disk image and give it a name that identifies its contents or version, such as Project-Reference-2026-10-15.dmg. In Image Format, choose Read-only image for a quick, uncompressed copy or Read-only Compressed to reduce storage use. Choose encryption only if you need password protection, and keep that password somewhere you can retrieve it. Click Save, then Done. These menu paths and image-format options are documented by Apple. Create a disk image using Disk Utility

The image mounts in Finder. Open it and confirm the expected documents are present; then eject it using the Finder sidebar. Keep the .dmg beside the project. When you need to consult the reference, double-click the image, view or copy out the needed material, and eject it when finished. Copying a reference file out creates a separate working copy; edit that copy if you need annotations or revisions, and leave the mounted reference image as your baseline.

Keep the live work separate and make updates deliberate

Do not save current working files inside the read-only image. Use the ordinary project folder for files that change, and label the mounted image or its contents with a clear reference or version name. If the approved reference changes, build a new image from the revised reference folder and retain or remove the older image according to your project’s retention needs. The old image will not refresh itself when source files change; that follows from creating a separate disk-image file from the folder.

A read-only image is useful for keeping a convenient stable reference, but it is not a backup or a guarantee that the file cannot be removed. Keep important project data in your normal backup process. If the goal is to stop a specific Mac user from changing a folder, Finder permissions offer a different control: in Get Info > Sharing & Permissions, set that user or group to Read only. Apple notes that permissions determine who can view or alter items, and that administrators may control access. Permissions are user access settings, not a substitute for preserving an independent reference copy. Change permissions for files, folders or disks on Mac

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