A lot of people often store sensitive information in portable storage devices, such as pen drive, external USB hard drives, etc. Sure, such devices make it easier to carry data on the go, but they are also vulnerable to theft. USB drives can easily get stolen, misplaced or get into wrong hands. And that’s why, you might want to lock your external drive with password to prevent unauthorized access to your data.

In a previous article, we have already covered different solutions for locking external USB drives. But most of the solutions mentioned there are primarily applicable to Windows users. So, if you’re a Mac user, follow the instructions below to protect your data on external USB storage.

Lock external drive with built-in software

Some of the popular external hard drive manufacturers like Sony, WD, Seagate or SanDisk provide native password protection apps for both Mac and Windows. If your external drive manufacturer provides such an app, it’s the best option as the solution is cross platform and you can use the password protection feature on both the OS (Windows and Mac).

If your hard disk manufacturer does not provide a password protection software compatible for Macs, you need to explore the alternative methods mentioned below.

Encrypt external hard drive on Mac

Fortunately, Mac OS X provides a few native solutions to help you protect your data on external hard drives. One option is to fully encrypt the external drive and lock it with password. But in order to do so, the external drive should be using the GUID Partition Table (GPT) scheme. If it uses the Master Boot Record (MBR) scheme, you might need to reformat the drive and change it to GPT scheme. This will, of course, erase all existing data on the drive. Also, an external hard drive that is fully encrypted in Mac won’t be accessible on a Windows computer at all.

Protect data using an encrypted disk image

If you frequently access your external drive on both Windows and Mac, the above solution might not be convenient for you. Instead, you can create an encrypted disk image and store it on your external drive. This will leave the drive accessible on both Windows and Mac computers, but the encrypted disk image can only be mounted on a Mac (with the correct password) to access its contents.

Create a new disk image on Mac

The following steps will allow you to create a new password protected disk image on Mac:

  • Connect your external hard drive to Mac via USB.
  • Launch Disk Utility from Applications directory.
  • Click on New Image from the top panel in Disk Utility.
  • In the popup that appears, provide the name of the image file and select the save location as your external drive. This will create the disk image on your external drive.
  • Further, you can give a custom name to the disk that will be mounted from the image, and choose its size and format.
  • To protect the disk with password, choose an encryption method. 256-bit AES encryption is more secure than 128-bit AES, but also takes longer to decrypt.
  • Set the image format as sparse bundle disk image. This will make the disk image as a dynamically expanding one.
  • Finally, click on the Create button to save the disk image on your external drive.

You’ll notice that the size of the actual disk image is much smaller than the assigned size. This is the advantage of sparse bundle disk image. As you mount the disk from the image and save files into it, the image size will automatically increase to contain the files. So even if you create a disk image of the same size as your external hard disk, the actual size of the disk image will correspond to the size of the data stored in it.

Mount encrypted disk image on Mac

Once the encrypted disk image has been created, the next step is to mount it on Mac as a virtual disk. To mount it, just double click on the disk image from your external hard drive. You will be prompted to enter the password. There’s also an option to remember the password in your keychain, but if you want to be prompted for password every time you mount the disk image, you should uncheck the option.

After the disk is mounted, you can use it as just another disk drive and copy data from and to it. If you delete any file or folder from the mounted disk, ensure that the Trash is cleared as well, otherwise the space occupied by the file won’t be freed up.

The image file dynamically increases in size as you copy contents into the mounted disk. Once the image size increases, it can’t be reduced even if files are deleted from the disk. But that shouldn’t be a concern as you’d get the resultant free space on the mounted disk.

Increase size of encrypted disk image on Mac

The disk image you have created is a dynamically expanding one, but it can only be expanded up to the size limit that was assigned during it’s creation. If you want to increase the capacity of the virtual disk, you need to resize the disk image. The following steps will guide you to do so.

  1. Double click on the disk image to mount the virtual drive (provide the disk password when prompted). This will ensure that the disk image is available in Disk Utility.
  2. Now launch Disk Utility and select the disk image from the left side.
  3. Right-click (Control-click) on the disk image and select Eject Disk Image from the menu. The Resize Image option should now be available for the disk image.
  4. Ensure that the disk image is selected on the left side, and click on the Resize Image button on the top panel.
  5. Provide the disk image password when prompted, and set the new size of the disk image.
  6. Click on Resize button to apply the changes.
  7. Now double-click on the disk image file and provide the password to mount the virtual disk.
  8. Back in the Disk utility, select the disk image on the left side and go to the Partition tab.
  9. In the Size text field, enter the resized value of the image or drag the white box till it fills the available space.
  10. Finally, click on Apply to save your changes. Now the mounted disk image will have the resized capacity.

We couldn’t find any feasible solutions that support external hard drive encryption on both the platforms (Windows and Mac). If you are aware of such solutions that were not mentioned here, let us know about it in the comments below.