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I got a new external SSD drive to use as a Time Machine on my new MacBook Air M1.

I formatted that new SSD with a single partition on the format of APFS Encrypted, and assigned a password.

I went to System Preferences > Time Machine in macOS Monterey 12.4. In the file picker to choose a drive, at the bottom of the dialog is a checkbox reading Encrypt backups, turned on by default.

👉 Is this encryption option within Time Machine redundant as I already have the APFS file system encrypted?

Is having both APFS encrypted and Time Machine encrypted harmless but costing in performance as everything is being encrypted twice?

What is the recommended configuration, APFS not-encrypted with Time Machine option "Encrypt backups" enabled?

Apple provides the support page, Keep your Time Machine backup disk for Mac secure. But it does not exactly address my question.

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Time Machine changes APFS to APFS (Case-sensitive, Encrypted)

In answer to the title question: Yes, the original encryption is replaced and hence redundant.

With encrypted APFS, the whole drive or even partition is not encrypted. The disk hierarchy (as viewed from Disk Utility) is 1) physical drive, 2) APFS container/partition, and 3) APFS volume(s). Encryption is for each APFS volume.

When choose an existing volume (encrypted or not) for Time Machine, it will erase the volume and create a new APFS volume with the same name. The new volume can, optionally, be encrypted.

Encrypting both is harmless, but it does not encrypt twice. Rather the Time Machine encryption replaces the first encryption. So there is no performance penalty.

And finally and as you suggest, the simplest way is to:

  1. Create a non-encrypted APFS volume.
  2. Choose that volume when prompted in Time Machine.
  3. Tick the Encrypt backups checkbox.

After Time Machine performs its first backup, choose File > Get Info in the Finder for that external drive, or examine in the Disk Utility app. You will see the format has been changed from APFS to APFS (Case-sensitive, Encrypted).

Just as a note, many of the "Related" links in the righthand sidebar are out of date as they refer to HFS+ formatted Time Machine disks.

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  • Good edits, Basil
    – Gilby
    Jul 10, 2022 at 3:10

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