With previous versions of of OS X , we were used to encrypt the drive by using the Disk Utility. However, with OS X Mountain Lion, the procedure can be done directly on the mounted drive, either from the Desktop or Finder.

1. Connect your External Hard disk, SD card, or USB disk to your MAC machine 
   
   
   

2. From the Desktop or Finder, right click on the External Disk and click on "Encrypt DiskName"... , where DiskName is the name of the External drive, in my case, my USB disk is named : External 8GB USB
   
   
   
   

3. You will be asked to type a password and confirm it. Also you are required to Type a password hint.
   
   
   
   If you want any help in choosing a complex password, then click on the Key icon , and the Password Assistant will launch. Where you can specify the type and length of the suggested password
   
   
   
   Once the password is set by typing it or by selecting a suggested password generated by the Password Assistant and a password hint is typed as well, Click on Encrypt Disk
   
   
   
   

4. Encrypting your disk drive will start, You can verify this by right clicking on the External disk and you will notice the the Encrypt "DiskName"... is now replaced with Encrypting "DiskName"...
   
   
   
   The process of encrypting the disk takes quite a long time depending on the disk size. Once the External disk is encrypted, it will disappear from your desktop and be mounted again. In this stage, your disk is now encrypted, to verify this, eject the disk and connect it again to your machine.
   

5. The next time you connect your External disk into your machine, before the device get mounted, you will be asked to enter the encryption password so that you are granted access to the external disk .
   
   
   
   If you forgot your password, you can click on the Show Hint button, so that you get a hint for your password
   
   
   
   Type your password and then click on the Unlock button. Congratulations, now your external disk is secured.

 

Note : Encrypting your External Disk on OS X Mountain Lion will only make it work and mounted on OS X Lion and OS X Mountain Lion. Trying to connect the encrypted disk to OS X Snow Leopard generated the below error message.

Summary

Encrypting your External Disk drive or USB thumb is a must to do procedure if you have files/photos on the disk drive and you don't wish anyone to have access to the drive in case it was stolen or lost. Its and easy process and a best practice process to secure your data.

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