Encrypt disks or memory cards to protect them
Using removable media, such as CDs, DVDs, removable disks, or memory cards, has become a popular way to transport information. However, if the removable media is lost or stolen, someone else can access its data by mounting the media on another computer.
If you want to keep private or confidential information secure on removable media, you need to encrypt the information on it.
In the Finder, open a window, then Control-click the item you want to encrypt in the sidebar.
Choose Encrypt [item name] from the shortcut menu.
Create a password for the disk and click Encrypt Disk.
Important: Be sure to record and keep this password in a safe place. You cannot access the data on the encrypted disk without it.
It may take some time to encrypt, depending on how much information is stored on your Mac. However, you can use your Mac as usual while the disk is being encrypted.
To decrypt an encrypted disk, Control-click the disk you want to decrypt, then choose Decrypt [item name] from the shortcut menu. For more information, see How does FileVault encryption work?.