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Encrypt your USB flash drive

Date: 29th February 2012     

USB flash drives (sometimes called thumb or pen drives), are perfect for carrying around files in a gadget that fits in your pocket. You can easily and conveniently carry work files, music, programs, utilities, and much more.

Their capacity ranges from a couple of gigabytes to over one hundred and some can store as much as a small hard disk drive. It's amazing when you consider their small size and tiny power requirements.

Although their convenience is beyond question, they have one serious flaw and this is that they are easily lost. Well, it is not really a flaw in the USB flash drive, it is a human failing.

We tend to lose small devices like USB flash drives and we drop them, we leave them in computers, they fall out of our pocket, and they mysteriously disappear into thin air when we swear we put them safely in our pocket.

A USB flash drive may contain valuable software, private files and other information we wouldn't want whoever finds your lost gadget to gain access to. Some sort of security system is required that enables the contents of the drive to be locked so that no-one can access it but you.

Securely encrypting the contents of a USB flash drive is actually quite easy when you know how, but it isn't obvious. Instead of encrypting the whole USB flash drive, you create an encrypted disk image and store it on the drive.

Go to the Utilities folder and run Disk Utility. Select the USB flash drive in the left-hand pane and then select the Erase tab. Set the format to Mac OS Extended (Journaled), enter a name for the drive and click Erase. It is not essential that the drive be formatted this way, but reformatting it means that Windows computers cannot access the contents of the drive. Since 90% of the computers in the world are Windows PCs, you have already prevented 90% of people from accessing your USB flash drive.

Format a USB flash drive

Just giving it an OS X disk format isn't enough though, so in Disk Utility go to the File menu and select New, Blank Image. Enter a filename in the Save As box and select the USB flash drive in the Where box.

Create a disk image

Click the Size and select Custom from the list of sizes that is displayed. You are prompted to enter the size. You need to enter a number that is just slightly smaller than the capacity of the drive. Finally, set the encryption to 128 or 256-bit AES. Actually 128-bit is fine unless you are carrying around government secrets. Now go ahead and create the disk image. You will be prompted to enter a password - don't forget it or you won't be able to access the files on the drive.

Encrypt a disk image

Using the securely encrypted USB flash drive is just like using any other disk image. Plug the device in to a USB socket, double click the device icon that appears on the desktop, then double click the disk image file to open it. Enter your password (it may be remembered for your account) and that's it. You can now copy files to and from it just like it was a regular disk drive. Unmount it when you have finished by dragging the disk image icon on the desktop to the Trash, just as you would with any other disk image.