Encrypt disks or memory cards to protect your Mac information
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.
WARNING: When you encrypt a disk or other media, the disk format is converted to the APFS format and then encrypted. Macs with older versions of macOS that don’t support the APFS format will not be able to read the device’s data. Consider whether you need to connect the media to a Mac with an older version of macOS before you encrypt the data. You can also update your other Mac to the latest version of the macOS that supports the APFS format.
In the Finder on your Mac, 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. See How does FileVault encryption work?