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Gary Robbins
User level: Level 1 (35 points)

How do I reformate my MacOS extended Time Machine backup disc to get ready for MacOS 28?

I got a notice after updating to MacOS 27 in settings that MacOS28 will not be compatible with my MacOS extended formatted ecrypted Time Machine backup disk. It referred me to the following bulletin:


Backup disks you can use with Time Machine - Apple Support


Which contains this note:

Compatibility: Some older versions of macOS created backup disks using Mac OS Extended as the file system format instead of APFS. If you have an existing Time Machine backup disk, and you chose to encrypt that backup, learn about support for encrypted Mac OS Extended disks in macOS 28 or later.

The linked document contains instructions to either decrypt the disc or convert the disc to AFPS format, however, it notes on either procedure that:

This solution doesn’t apply to encrypted Time Machine backup disks.

Which links back to the original bulletin. So I'm in a loop of service bulletins, and I'm left not knowing what to do. How do I address this?

Posted on Sep 18, 2026 12:44 PM

2 replies

neuroanatomist
User level: Level 9 (72,871 points)

Sep 18, 2026 1:15 PM in response to Gary Robbins

Time Machine is optimized for an APFS-formatted drive. Use Disk Utility to completely erase the drive, format it as GUID/APFS and apply encryption if desired. In Disk Utility, select View menu > Show all devices, then select the top level of the external drive and click Erase.

 

 

First set the Scheme to GUID then set the Format to APFS.



More info: Disk Utility User Guide for Mac - Apple Support

 

Since the disk will be used for Time Machine, it's better to skip encryption at this step and select the encryption option when setting up the Time Machine backup.

MrHoffman
User level: Level 10 (169,253 points)

Sep 18, 2026 1:21 PM in response to Gary Robbins

There are three general approaches, none being a great choice here:

  • Add a new HDD (or SSD) formatted as APFS, and continue to use the (decrypted) HFS+ target, operating in parallel until you have enough depth to erase the HFS+ (decrypted) storage and re-format it and re-use it, or to remove it and store it, or erase it and recycle it. The two Time Machine backup targets will alternate. Or run two Time Machine targets until you eventually replace the HFS+ HDD will a newer HDD (or SSD), and retire the HFS+ HDD.
  • Decrypt the HFS+ HDD volume and continue until such time as the HDD is no longer needed or no longer useful or no longer working, or some other such limit arises.
  • Erase the HFS+ HDD volume and format as APFS, and re-configure Time Machine and start over.


I’m not aware of a supported means to convert from HFS+ Time Machine to APFS Time Machine, though I have seen some creative attempts toward that goal. And even if that conversion were possible, I’d be concerned about something failing during the whole conversion process and losing some or all of the sole backup.


Time Machine works very differently between HFS+ and APFS.


Other potential options include adding a NAS box with Time Machine server capabilities. This works particularly well with laptops, as it means not needing to wire into the HDD. The Time Machine backups happen automatically when the laptop is in range of the same Wi-Fi with the NAS.

Originally posted at discussions.apple.com/thread/256360004