Do you want to give your computer to another person so they can use it, but before you give it away you want to remove all of your data? If so, here’s an easy way to accomplish this. We’ll create a user account for the person who’ll be using the Mac, then we’ll delete your user account.

These instructions were written for OS X 10.5 and 10.6, but should work on previous versions as well though some of the names and wordings have changed subtly.

• Turn on the Mac and let it start up.
• Go to the Apple menu, select System Preferences and click on Accounts
• If the padlock in the lower left corner is locked then click on it to unlock it. Enter your user account’s password when prompted to do so.
• Click the plus sign button to add a new user account. Set “New Account” to “Administrator”. Enter the real world name of the new user of the Mac in the “Full Name” field. Enter a name in the “Account name” field; this name can not contain spaces, capital letters or punctuation marks. Enter a password in both the “Password” and “Verify” fields. Enter a password hint if you’d like.
• Click the “Create Account” button.
• Go to the Apple menu and select Log Out which appears at the bottom of the menu.
• You’ll be presented with a login window. Type in the account name and password for the account which you just created. You’ll be logged into this new user’s account and now that you’re no longer using your account it can be deleted.
• Go to the Apple menu, select System Preferences and click on Accounts
• If the padlock in the lower left corner is locked then click on it to unlock it. Enter your user account’s password when prompted to do so.
• Click on your user account which’ll be listed on the left-hand side of the window.
• Click the minus sign button to delete your user account.
• Select “Delete the home folder” to have all of the data in your user account deleted.
• Click the OK button. The actual deletion process could take 10-15 minutes, especially if there is a lot of data in your account.
• When the deletion process is complete you can close the System Preferences window.

This post builds upon the information that I presented in my last blog entry about how to securely store personal information on your Mac. A disk image file is a special kind of file. When a disk image file is opened it makes the computer think that an actual disk, like a CD, has been physically inserted into the computer. Thus, a disk image file, or more simply a disk image, can be thought of as a virtual disk.

How to create and use a password-protected (encrypted) disk image in Mac OS X 10.4, 10.5 or 10.6

1. Go to your Applications folder and open the Utilities folder.

2. Open Disk Utility (/Applications/Utilities/).

3. Click the “New Image” button, or choose New > Blank Disk Image from the Disk Utility File menu.

• Enter a name for your disk image file in the Save As field

• Change the save destination to either your Documents folder or your Desktop, whichever you prefer.

• Set Volume Name set to “Virtual Disk” (In 10.6 the field is called “Name”, not “Volume Name”)

• Select a Volume Size for the image file. If you’re just going to store a few Word or Excel files in the disk image then 10 or 20 MB should be large enough. (In OS X 10.6 40 MB is the smallest you can select.) You can choose any size you’d like by selecting “Custom” at the bottom. If you ever fill your disk image you can always create a larger one and move the files from the smaller disk image to the larger one.

• Leave the volume format set to “Mac OS X Extended (Journaled)” (In 10.6 the field is called “Format”, not “Volume Format”)

• Leave “Partitions” set to “No partition map”

• Leave the Image Format set to “read/write disk image”

• Set Encryption to “128-bit AES” if your Mac uses Mac OS X 10.4. Choose “256-bit AES” if your Mac uses Mac OS X 10.5 or 10.6

Warning: If you forget the password to your encrypted disk image, your data will be irretrievably lost so please write down your password on a piece of paper. This is just a precautionary measure.

• Click the Create button.

• Important: In this next step you’ll need to first uncheck “Remember password (add to keychain)” then twice enter a strong password. This password is used to secure your disk image.  [By "strong password," I mean one this is long and contains a mix of letter, numbers, odd capitalization and punctuation marks or at least most of these features. For example, 1%milKisgooDforyoU is a strong password.]

• Click OK

4. The disk image file will be created and then it’ll be opened automatically.

5. Look at your Desktop you should see an icon named Virtual Disk. This is what appears whenever your disk image is opened. Copy a file into this Virtual Disk. Notice that the file will automatically be copied when you put it in the Virtual Disk. The original file will remain where it is so you’ll probably want to put it into the Trash in a couple of days after you’re sure you understand how your new disk image works.

6. Close Virtual Disk by dragging its icon to the Trash, which will eject this virtual or fake disk.

7. Now, locate your disk image file. (It’s in the location that you selected in step 3, second bullet). Double-click your disk image file. Enter your password when prompted and then click the “OK” button. You should now see the icon for Virtual Disk again. Open it and you’ll see the file that you copied into it.

8. Now that you know how to use your disk image you can copy more files into it and you can delete the originals. For example, if you keep a list of passwords or other sensitive information in a Word or Excel file, copy this file into the secure disk image. Once you’ve confirmed that this file is inside your disk image then throw away the original file so it’s no longer sitting unprotected on your Mac’s hard drive.

9. Don’t leave your Virtual Disk on the Desktop all day long. That defeats the added security. Only open it when you need it and then close it when you’re done by dragging its icon to the Trash icon on your Dock.