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Password Protect Files: Apple OS X

Updated on February 1, 2016

Apple users can password protect files without downloading, installing, or purchasing any special software. Disk Utility, a disk management tool that comes with Apple computers running OS X, can create an encrypted disk image (.dmg). Users can choose a 128 bit or 256 bit encryption.

Opening an encrypted disk image requires the password (unless the password has been stored in the Keychain), making it a safer way to send sensitive files via the Internet or other network. The same applies to storing sensitive files on a laptop or portable storage device. If the device is lost or stolen, the person finding it would have the disk image, but not the password.

Sizing Up The Files

Determine the total space needed to store the files. Check the size of a folder or individual file by highlighting the folder or file, then choose Get Info under the Finder’s File menu. Alternately, highlight the file or folder, then use the shortcut Command + I.

But there's another, more clever type of disk image. Choosing Sparse or Sparse Bundle as the image format creates an expandable disc image. Sparse Bundle is the more modern if you will, and stores data in bands rather than a single file. Sparse Bundle is also more efficient in some backup scenarios with the downside being compatibility with older OS X versions (10.5 and earlier).

Why use a Sparse or Sparse Bundle? A normal 100MB disk image will always take up 100MB of space. A Sparse or Sparse Bundle 100MB image filled with 20MB of data will occupy closer to 20MB of space, but is capable of holding up to 100MB of data. Quite clever right?

Regardless of the image format, choose an image size larger than the total of the files. Consider the file types you might be storing when determining the disk image size (or maximum size).

Creating The Disk Image

- Open Disk Utility (Go menu/Utilities).
- Choose New Image.
- Name the disk image file (.dmg) in Save As.
- Name the disk image. (displayed when the image file is opened/mounted).
- Select a size.
- Change the format or partition map if desired.
- Select an encryption setting (128 bit or 256 bit). Otherwise the image is not encrypted.
- Click Create.
- Enter a password.
- Deselect "Remember password in my keychain" to require the password to be entered on this computer.
- Consider storing a copy of the password in a safe place in case you forget it.

Fill It Up With Files

- If the disk image is not mounted (opened), double click the disk image file (.dmg).
- When prompted, enter the password. The disk image should then mount like an external hard drive, flash drive, or other such device.
- Drag the desired files or folders into the open disk image window.
- Eject the disk image (icon).
Dragging files to a disk image copies the files instead simply moving them. Once you have verified a copy in the disk image, delete any files you do not wish to store in the original place. Don't rely on the disk image as your only copy of important files, keep multiple copies in safe places.

Sending or Storing the Disk Image File

- Transfer/send the disk image file (.dmg) as you would any other file.
- Store the disk image file in Documents or other location of your choosing.


Dragging files or folders into a disk image does not move the files or folders, but makes a copy. Sensitive files could remain open for access unless you delete the copies on the computer outside of the disk image. Before deleting any files, make sure you have an entirely separate backup copy – on an external drive or flash drive for instance. It is important to have at least two copies of important files at all times.

Refer to the Apple support site for more about the Sparse format and other settings options as well as subtleties between versions of OS X.


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