How to format a new Time Machine HDD as APFS Case Sensitive on Mac?

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growler62000 Author

Level 2

195 points

How to format a new Time Machine HDD as APFS Case Sensitive on Mac?

New Mac Studio M4 Max. (I had Late 2014 27” iMac running Monterey)

While deciding which new HDD to buy for Time Machine, I used my old Seagate HDD, 5 TB, T.M. Drives (formatted as Mac OS Extended (HFS+) as my temporary T.M. HDD.

I used Disc Utility to erase & format as Mac OS Extended (HFS+). (Even though it already was.)

I've been let it do the first Time Machine back up. It backed up without issue except when I checked it in disk utility… It now was APFS!

I did some searching and found that Mac now requires new Time Machine backup HDDs to be formatted APFS Case Sensitive!

So it seems it it pointless to format a new HDD as Mac OS Extended (HFS+)

I read a similar post here from 2023, just now re Ventura.

So the idea that you should use Mac OS Extended (HFS+) for HDDs & APFS for SSD apparently no longer apples when using the drive for T.M.

[Re-Titled by Moderator]

Mac Studio

Posted on May 8, 2025 12:51 AM

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Answer 1

Top-ranking reply

VikingOSX

Level 10

124,760 points

May 8, 2025 5:17 AM in response to growler62000

You can format a Time Machine intended drive as case-insensitive and Time Machine will still mount it as case-sensitive. It has a mind of its own…

A Get Info of the mounted Time Machine drive will show you that it is mounted (case-sensitive) even when formatted as just APFS (implicitly case-insensitive).

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Answer 2

May 8, 2025 2:33 AM in response to growler62000

To format a new Time Machine HDD as APFS Case Sensitive on Mac, open Disk Utility, select the drive, choose "Erase," and then select APFS as the format and Case-Sensitive as the scheme.

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Answer 3

dialabrain

Level 10

136,715 points

May 8, 2025 5:05 AM in response to growler62000

Yes. Time Machine new drives are now automatically formatted as APFS/Case Sensitive.

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Answer 4

Barney-15E

Level 10

123,308 points

May 8, 2025 2:31 AM in response to growler62000

So the idea that you should use Mac OS Extended (HFS+) for HDDs & APFS for SSD apparently no longer apples when using the drive for T.M.

The issue with HDD is that APFS doesn’t optimize writing to contiguous storage space. It will let the drive controller write parts of the file anywhere which would slow access speed. However, files only get broken up into extents when a large file is edited and grows larger than what it originally occupied. The file gets fragmented. For Time Machine, files are not edited. When originally written, they are all contiguous and they are never edited.

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Answer 5

growler62000 Author

Level 2

195 points

May 8, 2025 5:01 AM in response to growler62000

The moderator re-titled my post.

I never asked how to format a new T.M. drive as APFS.

It is obvious.

My original title was:

“HDD formatted Mac OS Extended (HFS+) is changed to APFS by Mac after 1st T.M. backup”

I did not ask any question.

I was stating a fact that I just learned.

I posted to see if anyone disagreed with me or I had somehow misunderstood what was going on when I formatted a HDD as Mac OS Extended (HFS+)… successfully & then after the very 1st T.M. backup, it was automatically changed to APFS Case Sensitive format.

I think the last 2 posts were people being helpful & answering the Moderator created question/title.

RE Barney’s reply, I think he was explaining WHY Apple does not allow you to use Mac OS Extended (HFS+) as the file format for a T.M. backup.

But, I’m not sure. Barney, if I misunderstood your post, please clarify.

Thanks!

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Answer 6

dialabrain

Level 10

136,715 points

May 8, 2025 1:12 AM in response to growler62000

Select the HDD in the Finder Sidebar, right-click on it and choose "Erase Disk…", then choose APFS & Case Sensitive.

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