Rob Young

There are bound to be files on your computer you’d rather keep private such as files for work, financial information or simply things you shouldn’t have in the first place.

If you’re new to OSX and want to guard certain information against being seen in the eventuality of theft or otherwise, this short guide may help.

To secure files and folders, you’ll need to create an encrypted disk image – much like the ones commonly seen when downloading applications from the internet.  This will encrypt the contents of the disk image with a password you provide, and require that in order to decrypt and view the contents, one will need to provide the password.

To create an encrypted disk image:

1. Open the “Disk Utility” application
2. Go to File > New > Blank Disk Image
3. Choose a name and location for your disk image
4. Choose a size for the disk image.*
5. Choose AES-128 encryption, or 256 bit if you wear a tin hat (reduces performance)
6. Choose “sparse bundle image” from the Format selection drop-down.*
7. Click create, and enter a secure password. If you’re stuck for a password, the assistant is very useful.
8. Done!

Your disk image will now be saved to the location you specified in 3. When you double click it you’ll be prompted for your password before it is mounted, at which point it’ll appear on your desktop and at the side of every finder window.

Don’t forget to eject the image when you’re done with it

* The size you choose here will be the maximum capacity the disk image will be able to hold. Some presets are provided for the size of a CD or a DVD should you wish to transfer the contents to removable media when it becomes full. Note if you choose a sparse image type, the disk image will only consume as much space on your hard disk as its contents if less than full, not the size you specify here.

** A sparse image is one that will only consume as much disk space on your computer as the items inside it, so it makes sense to choose this format. You’ll notice there are two different types of sparse image available.  The difference between a sparse disk image and a sparse bundle image is how the file is stored on your hard disk; the sparse disk image is one large file whereas the sparse bundle is split into smaller blocks. This means that should you add a small file to the disk image, backup utilities (Time Machine most notably) will only need to back up that “block”, and not the whole disk image file. This saves time during backups, and also disk space on your backup volume when using Time Machine, especially when the disk image is a large one.