Encrypt and protect a disk with a password using Disk Utility on Mac
When you format an internal or external volume, you can encrypt and protect the volume with a password. If you encrypt an internal disk, you must enter a password to access the disk and its information. If you encrypt an external device, you must enter the password when you connect the device to your computer.
Important: If you encrypt an external drive, you can’t connect it to an AirPort base station for Time Machine backups.
To encrypt and protect the volume with a password using Disk Utility, you must erase the volume first. If the volume contains files you want to save, be sure to copy them to another volume.
Important: You can encrypt your data without erasing it by turning on FileVault in the Security & Privacy pane of System Preferences (see Encrypt Mac data with FileVault). You can also encrypt a disk and protect it with a password without erasing it (Control-click the disk in the Finder, then choose “Encrypt disk name.” See Encrypt disks or memory cards to protect your Mac information.
If the disk contains any files you want to save, copy them to another disk or volume.
In the Disk Utility app on your Mac, choose View > Show All Devices.
In the sidebar, select the disk you want to encrypt.
Click the Erase button in the toolbar.
Enter a name for the volume.
Click the Scheme pop-up menu, then choose GUID Partition Map.
Click the Format pop-up menu, then choose an encrypted file system format.
Enter and verify a password, then click Choose.
(Optional) If available, click Security Options, use the slider to choose how many times to write over the erased data, then click OK.
Secure erase options are available only for some types of storage devices. If the Security Options button is not available, Disk Utility cannot perform a secure erase on the storage device.
Writing over the data three times meets the U.S. Department of Energy standard for securely erasing magnetic media. Writing over the data seven times meets the U.S. Department of Defense 5220-22-M standard.
Click Erase, then click Done.
To change the password, select the volume in the sidebar, then choose File > Change Password.