Time Machine disk no longer encrypted and not working

Tahoe quietly flipped FileVault on during the OS upgrade, and now Time Machine is refusing to keep appending backups to a disk whose encryption state no longer matches the source volume. That “disk is no longer encrypted” message is not saying your old backups vanished, it’s saying the security model no longer lines up, so macOS blocks further snapshots.

Your existing data on the Time Machine disk is still readable right now if you mount it in Finder or browse it via Disk Utility, but once FileVault and Time Machine get out of sync there is no supported way to re-encrypt that backup set in place without invalidating it. Forgetting and re-adding the disk sometimes clears the flag, but in practice Tahoe usually keeps rejecting it because the snapshot chain was created under a different crypto state. That’s why wiping and starting fresh worked for you, and it’s also the safest path if you care about integrity.

If you ever hit this again, the right move is first copy anything you need out of the old TM volume while it still mounts, then erase it, enable encrypted Time Machine, and let macOS build a clean, consistent backup set going forward.

What is the specific text of the warning? Did you recently upgrade to macOS 26 Tahoe? Doing so enables FileVault by default, and if you were previously backing up to an unencrypted Time Machine destination disk but your internal storage now has FileVault on, you may be seeing a warning about backing up an encrypted Mac to an unencrypted TM backup.

If that's the case, you can either turn FileVault back off (System Settings > Privacy & Security > FileVault) or erase and restart your TM backups as encrypted (use Disk Utility to erase the external drive and format it as GUID/APFS without encryption, then when setting up the new TM backup choose the option to encrypt it and select a password).

Personally, I'd choose the latter approach. My Mac has private data on it (financial data, tax returns, all the stuff identity thieves love) so using FileVault to protect the internal storage and having an encrypted backup are important to me.

Note that if your Mac can run Tahoe then it has either a T2 chip or Apple Silicon, meaning the internal storage is already encrypted. FileVault incorporates your login password into the cryptographic key. That's important because if someone gains physical access to your Mac and FileVault is off, they can reset your login password and access most of the data on your Mac (your keychains and logins/passkeys in the Passwords app are not accessible, nor are data stored in encrypted disk images, but everything else is an open book).

Hi Flybr1dge,

I encountered the same problem, and I solved it by turning FileVault off and then going into the TimeMachine settings. Deselect the external TimeMachine drive (not erase just highlight it and press -), press "Forget drive" and then press + to select it again - that should work:-)

Snedkerjan

You can usually recover your existing backups safely. Run First Aid, check the encryption status in Disk Utility, re-select the disk in Time Machine, and try re-enabling encryption. Only erase the drive if these steps fail — most of the time a full wipe is not necessary.

Thank you Very Much. Worked just as you explained.