OS X El Capitan lets you format a USB memory stick so that its contents are encrypted. You’ll need to enter a password whenever it’s inserted.
Essentially this turns any USB stick into an ultra-secure portable file storage device, of the type often sold at a premium. However, bear in mind that the memory stick will only work on Macs also running OS X El Capitan and not on Macs running earlier versions of OS X or on PCs running Windows or Linux. To those operating systems, the memory stick will appear to be unformatted or corrupted.
For this tip to work, you’ll need a memory stick of any size. Be aware that files already on the stick will be deleted during the formatting process, so you should temporarily copy them to a safe location and then copy them back once the following procedure is finished.
- Start by opening Disk Utility (Finder->Applications->Utilities->Disk Utility) and then insert the USB memory stick you intend to use.
- Look for the memory stick’s entry in the list of disks on the left side of the Disk Utility window. It will probably be identified by its size. Select the entry, but make sure that you select the disk itself and not the partition(s), which will be listed below and indented slightly.
- Click the Erase tab in the Disk Utility window. In the drop-down menu alongside the Format heading, select Mac OS Extended (Journaled, Encrypted). In the Name field, type whatever you wish to call the memory stick. This name will appear in Finder’s sidebar whenever you insert the stick in future.
- Click the Erase button. You’ll be prompted to enter a password and verify it by typing it again immediately below. It’s important that you don’t forget this password! If you do there is absolutely no way of recovering the contents of the memory stick—they’re lost forever. However, you will be able to reformat the memory stick so you can keep using it although the contents will obviously be lost. Because of the risk, it’s a good idea to type something in the Hint field that might provide a clue to what the password is—the hint will appear in future should you get stuck when entering the password.
- When you are done, click the Erase button in the dialog box that should have appeared. Erasing, partitioning, and encrypting will take a minute or two depending on the size of the memory stick. Once you’re done, the new memory stick will be ready for use. You can copy files to it by selecting its entry in the sidebar of Finder. You can also close Disk Utility.
From now on you can use the encrypted memory stick just like any other memory stick. Before physically unplugging it, be sure to eject it by clicking the Eject button next to the disk’s entry within Finder.
When you insert the memory stick, you’ll be prompted for the password. If when prompted for the password, you put a check in the box next to Remember the Password in My Keychain, you’ll never be prompted for the password again on that computer. However, if it’s inserted into another Mac, the password prompt will appear. Therefore, you’ll have a USB memory stick that—essentially—works seamlessly on your computer but whose data is inaccessible to anyone else.
Note that the steps outlined in this tip are not limited to USB memory sticks. You could encrypt an entire portable USB, Firewire, or Thunderbolt hard disk in this way, making it so that the contents will only be accessible upon entry of a password. Bear in mind that accessing an encrypted drive might be a tiny bit slower. However, this effect is only likely if you regularly save large files such as movies there.