How to easily encrypt external drives in OS X Mountain Lion

How to easily encrypt external drives in OS X Mountain Lion

OS X has long let you encrypt external drives, and even your Mac HD. Mountain Lion has actually made it even easier to encrypt external disks with just a few clicks. It's sometimes a good idea to do so on items such as backup disks. This way, if they ever fall into the wrong hands, no one should be able to access your data without the password you have set to decrypt it.

If you aren't sure how to encrypt your data, follow along and we'll show you how.

  1. RIght click on the external disk on your desktop that you'd like to encrypt.
  2. Now click on Encrypt 'disk name'.
  3. You'll now be asked to choose a password that you'll need to gain access to encrypted content. Do not lose this password! If you do, you will not have access to the files on the disk. We suggest writing it down and storing it in a safe place or using an app like 1Password to keep it safe for you. We also recommend generating a password that is over 10 characters long (minimum).
  4. Once you've chosen your password and password hint, now click on Encrypt Disk.

That's all there is to it. Your Mac will now start encrypting the disk that you've chosen. This process could take quite some time depending on how much data you've got stored on it. Right clicking will show you if the disk is still encrypting.

As a side note, anytime you connect this external drive, you'll be asked for the password you set up unless you save it in your Mac keychain. Just be careful not to save this password to any Macs that you don't want to always have access to the drive when connected.

Have something to say about this story? Leave a comment! Need help with something else? Ask in our forums!

Allyson Kazmucha

Senior editor for iMore. I can take apart an iPhone in less than 6 minutes. I also like coffee and Harry Potter more than anyone really should.

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Reader comments

How to easily encrypt external drives in OS X Mountain Lion


Thanks for the tip Ally! I'd add that every time you connect the disk, Finder will prompt you for that password. Unless you add the password to Keychain, which will automatically enter the password for you. (Of course, you should only add the password to Keychain on Macs that you use often.)

The password used in the example (10 characters) is too short for anything intended to remain private. Weak passwords incapacitate strong encryption in 9 out of 10 cases -- or more.

My actual password is over 20 characters long. I did however add to the article to use something over 10 characters. Thanks for pointing it out.

I should do this ... whenever we leave for more than a day I stash my external HDs and Mac Mini somewhere safe... but this would give a bit more security just in case!

This seems to be a GREAT idea. But, one item to which you do not speak. If the external drive is connected to my MacBook Pro as the device to which I direct all documents, iTunes music, books, apps, etc., and movies, what occurs when I update a document (e.g., an Excel spreadsheet), or I purchase a new album? Am I directed to use a password to un-encrypt the drive each time? Or, does the drive encrypt itself whenever it is turned off, or disconnected from my MacBook Pro?

How do I set this process up so that it does not destroy any effective and efficient workflow, while still achieving the desired goal of encryption? Thank you.

You only need to provide the password once, when the external drive is first connected. And, if you use it often enough, you should add the password to OS X's Keychain so it will automatically provide the password for you. The data on the disk remains encrypted whether or not it's connected.

But, on every read / write operation to the encrypted disk, the CPU needs to actually decrypt / encrypt data at some point. (Note: this info is all from Wikipedia's "File Vault 2" page.) The file system reads encrypted data from the disk, then some part of the OS kernel or file system code decrypts it. Likewise, the OS kernel or file system encrypts data and the file system writes it to disk in encrypted form.

This happens transparently, but it does cause a performance hit. Macs with "Core i" CPUs will lose about 20-30% of read / write performance, and older Core CPUs will lose more. The encryption is done in the CPU with Intel's AES (Advanced Encryption Standard) instruction set.

Hello, I'm glad this article was the top hit on Google. 1 thing I noticed with external drive though, I followed the instructions and after I created a password, it gave an error that the drive must use GUID Partition Table (mine is the older MBR - Master Boot Record). I found a post - - of a way to convert the table with gdisk utility, but that also gave me an error due to overlapping partitions. And I couldn't resize the 1 partition because it was MBR... My only recourse is to wipe the drive (backup data first), and create new GUID layout with Disk Utility.

Thanks for the article. This worked for my Time Machine drive, but it fails on my Clone drive because it's looking for a recovery partition.

Any suggestions?

What happens if, while accessing your encrypted disk you pull out the portable hard drive, or when there is power failure, will the disk stay encrypted?


Perhaps this is a silly question but if Finder is what prompts the password on a mac is there something set up to do the same when you plug the drive into a PC? I need to encrypt my hard drive to be readable by both mac and PC.