Encrypt a Time Machine disk without destroying the only backup

A Time Machine backup disk can contain nearly everything on a Mac. If the disk is lost, encryption helps prevent casual access. But moving from an unencrypted network backup to an encrypted one can erase existing backups, so the migration needs a recovery plan first.

Before you start​

Confirm whether the destination is a directly attached external disk or a network/Time Capsule destination. Open a sample file from the existing backup and maintain another usable copy before any conversion.

Do it step by step​

1. Open System Settings → General → Time Machine and identify the current backup disk. Record the latest successful backup date and the disk's physical location.
2. For a directly attached disk, follow Apple's Finder procedure: Control-click the backup disk and choose Encrypt when offered. Read the exact prompt rather than formatting or erasing it.
3. Create a strong backup password, store it outside the Mac and disk, and choose a hint that does not disclose the secret. Apple says the password is required to restore encrypted files.
4. For a network destination, stop before switching blindly: Apple warns that changing an unencrypted network backup to encrypted may erase the old backup set. Establish a second backup first.
5. After the operation, reconnect the destination, enter the password if required and run a new backup. Browse Time Machine and restore a harmless test file to a temporary location.

Check the result​

The disk should unlock with the saved password, new backups should complete and a test file should restore. Keep the recovery password accessible without the original Mac.

If something goes wrong​

If encryption does not complete or the disk reports errors, preserve the existing backup and consult Apple's disk guidance. Do not erase the only recoverable copy to make the setting look tidy.

Know the limit​

Encryption protects confidentiality, not against drive failure or lost passwords. Apple distinguishes local and network conversion behavior and warns that the backup password is necessary for restore. Apple's backup-disk encryption guide Apple's backup password explanation