An attacker can copy large amounts of sensitive information from the USB flash drive of an unsuspecting victim in just a few minutes; encrypt the drive to prevent this from happening to you. Windows 8.1 Pro and Enterprise have a built-in encryption system that you can use to password-protect your flash drive, even if the drive already contains data. BitLocker uses 128-bit keys and the Advanced Encryption Standard encryption algorithm to protect your data. Be aware, however, that you may lose everything on the flash drive if you can't remember the password and don't have a recovery file.
Press "Windows-X" to expand the Windows 8.1 Power User menu and select "File Explorer" to launch the utility. Alternatively, right-click the Windows button to display the Power User menu.
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Right-click the USB flash drive in the left pane and select "Turn on BitLocker." Don't remove the drive from the computer while BitLocker initializes it.
Check the "Use a password to unlock the drive" box and then type the password into the "Enter your password" and "Reenter your password" fields. Click "Next."
Select the "Save to a file" option to back up the recovery key in a text document on your hard drive. To print the key, select "Print the recovery key" and follow the instructions. If you click "Save to your Microsoft account," Windows stores the key in a document in your OneDrive account.
Type a name for the key into the "File Name" field, select a folder from the left pane and then click "Save" to save the key. Click "Next."
Select the "Encrypt used disk space only" option if your drive doesn't contain any data. New data is encrypted as you add it to the drive. If the drive already contains data, select the "Encrypt entire drive" option. Click "Next."
Click "Start encrypting" to encrypt the flash drive. Don't disconnect the drive from the computer while Windows encrypts it; you may lose valuable data if you disconnect it. Your files are not protected until the encryption is complete.
Unlock the drive by right-clicking it in File Explorer and selecting "Unlock Drive."
Type the password into the field and click "Unlock" to unlock the drive. All your data is available as soon as you unlock the drive, so you can rename, copy, move, add or delete files.