Best Disk Format for Time Machine on Monterey

I’m running macOS Monterey (12.1) on an ancient iMac (Retina 5K, 27-inch, Late 2015, iMac17,1; shipped with 10.11 El Capitan and all newer OS installs have been updates, NOT clean installs).

→ About This Mac → System Report → Hardware → Storage reports:

Two Internal Drives used for macOS, applications, and data:

  1. 121GB SSD formatted as APFS (64GB free)
  2. 6TB SATA Spinning Rust formatted as Journaled HFS+ (3.6TB Free)

One External USB3 Drive used for Time Machine continuously for over five years:
4TB Spinning Rust formatted as Journaled HFS+

Main Question:
Would my Time Machine backup be more robust if the external drive were formatted as APFS? As I understand, APFS supports snapshots (like ZFS) so I was thinking that Monterey TM would make use of snapshots for its backups instead of hard links like TM used in the past. Is a Monterey Time Machine backup more robust on a drive formatted as APFS (vs. HFS+ Journaled) or not?

Bonus Points:
Would you recommend any utility to clean off old cruft from prior operating system versions?

I would not screw with the 4TB external. It has a lot of your backup history on it. You shouldn’t expose that to risks connected with an attempt to change its formatting. Also, APFS indeed performs best on SSDs (it works on HDDs too despite some stories being passed around, it’s just not best suited to HDDs).

I would, however, buy a new disk (preferably an SSD if you can afford it) for TM and on that I would indeed use APFS. TM on APFS (“TMA”) offers several advantages over HFS+ (“TMH”). These three links to Howard Oakley’s site offer a very nice summary of why TM on APFS is what you really want.

TMA appears to be considerably quicker and more efficient than TMH, and doesn’t seem prone to choking problems which have troubled recent versions of TMH.

The end result is, in almost every respect, superior to TMH, and Apple rightly recommends users to prefer it.

If you’re using Time Machine in Big Sur and are still backing up to HFS+, you might wish to reconsider that decision.


It is not possible to convert an HFS+ Time Machine volume to APFS. Time Machine on HFS+ uses massive amounts of hard-links to directories - a feature that APFS does not support. In order to change the volume to AFPS, you will (not risk) lose all the data currently backed up there. This doesn’t matter for everybody, but if it matters to you, it’s a critically important fact.

From what I’ve read (mostly articles from Howard Oakley), yes. APFS’s snapshot mechanism is more robust than hard links on HFS+. It will also prevent malicious (or accidental) deletion of files from backups, because snapshots are all read-only.

If you stick with a HDD, you may see slower performance with APFS, but this may not be a big deal for Time Machine. APFS’s logic will create a lot more fragmentation on the disk, but since Time Machine only deletes files as a part of whole-snapshot deletion, it’s not going to be as bad as if you used an APFS HDD as storage for the files you’re actively using.

Of course, an SSD will work much faster, but SSDs cost more per TB and are not available in high capacities (very few larger than 2TB and none larger than 4TB, as far as I know). So you need to decide for yourself if it’s worth using.


You say:

I would not screw with the 4TB external. It has a lot of your backup history on it.

First, I wasn’t very clear, was I?

  1. I was referring to the internal 128GB SSD where macOS is installed when, in the Bonus Question, I asked about removing old files left behind after successive OS upgrades.
  2. Also, I wasn’t suggesting changing the disk format in place. I had in mind getting a new drive, formatting it as APFS, and starting a new Time Machine (TM ) backup.

Secondly, you make me wonder: is it possible to move/import my existing, multi-year HFS+ TM history to a new (empty) APFS TM drive? (Sounds too good to be true, but I have to ask.)

Thank you very much for the links. I did some searching for Monterey TM on HFS+ vs. APFS drives, but didn’t find much.

I’ll give these a read.


Thank you for your thoughts on Time Machine (TM ) on HFS+ vs. APFS.

As you pointed out, the downsides of APFS on a HDD vs SSD, i.e., fragmentation on deletes and read/write speed, are not worth the price premium. These disadvantages aren’t going to be even noticeable for TM.

Wirecutter is recommending the Seagate Backup Plus Hub and Amazon has a 10TB version for less than $300.

I think I’m all set.

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You can indeed move a TM store from one disk to another. It’s a bit involved, but it works. I’ve done it myself. However, you cannot change the formatting when you do so: TM works very different with HFS+ disks (hard links) than it does when using APFS (snapshots). See Howard’s articles I linked to on that.

So if you would like to benefit from TM to APFS you should get a new drive and start there from scratch. BS or Monterey TM will start using the new APFS disk in the modern “TMA” mode as soon as you tell it to use your new APFS disk. You can retain your old HFS+ in case you ever need to restore a file (or version of that file) from the time before you migrated to the new TMA. Unlike TMA, TMH leaves you with a file structure on your TM drive that you can navigate in the Finder as if it were any regular file storage.

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If you really want to get a HDD, I suggest not buying an all in one, but instead getting a disk and an enclosure. This way you know exactly what you’re getting, you can change/swap either independently of the other, and finally, it’s also usually less expensive this way.

Since you’re looking for a TM backup drive, performance is not crucial so SATA over USB3 and 5400 rpm is sufficient. Also TM use is rather modest (hourly, usually limited amounts of data) so you don’t need to worry about active cooling and all that mumbo jumbo.

You can get 4TB for $103 this way without skimping and sticking with reputable manufacturers.

Of course best performance, least noise, and the all around better user experience comes with an SSD. But it’s obviously much more pricey. This combination here is $393 for 4TB. This price will buy you the full APFS TM on SSD experience though. Your call as to if that is worth that much to you.

I got fed up with HDDs a while ago so by now all my running TM setups rely on SSDs. It costs some, but I’m happy with these sleek, silent, and quick backup solutions. I’m never going back to HDDs.

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Unfortunately, this isn’t possible. Due to the fact that Time Machine on HFS+ (TMH) is structured completely different from Time Machine on APFS (TMA), you can’t just convert a volume from one to the other.

I suppose it would theoretically be possible to copy-convert a TM volume. You would have to do something like:

  1. Copy the oldest backup from the TMH volume to the TMA volume
  2. Create a new snapshot on the TMA volume
  3. Copy the next backup from the TMH volume to the TMA volume. But you can’t just copy everything. You will want to compare the files so you don’t overwrite unchanged files, and you will want to only copy those disk blocks that differ when there are changes, in order to take advantage of APFS’s deduplication capabilities.
  4. Go to 2. Repeat until done.

But this would take an extremely long time to run. You’d effectively be running the Time Machine backup logic hundreds (or thousands) of times.

As far as I know, nobody has released a software package to do this. And even if one existed, I don’t think I’d want to use it anyway.

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The “lightbulb moment” for me when I posed a similar question a few weeks ago (and received the sage advice of @Shamino and @Simon they’ve kindly repeated here) was when I realized I had thought about TM backups wrongly. Unconsciously, I believed they were sacrosanct artifacts of all my precious data going back eons that must be preserved in perpetuity. The reality was that HDs have a limited lifespan, new ones are cheap, and my data only went back about six months anyway (the drive was full). If I bought a new drive and simply started over, I just keep the old drive on the shelf, no longer subject to wear, and hook it back up should I need to restore anything it contains.



Yes, that’s the message I’m getting too. I really didn’t think about my existing TMH HDD being at the end of its useful life. 🤦🏻

For what it’s worth, I DID search TidBITS Talk for other discussions of TM on APFS vs HFS+, but didn’t find anything. (And I posed the question on which was utterly useless.)

@Shamino @Simon

Sorry to put you through explaining all this a second time.

There’s no reason to apologize at all.

TM has gone through tremendous changes since Catalina. Similarly, the way macOS sets up its boot volume and separates user data from signed and sealed system data has changed radically how we need to think about things such as backups, cloning, emergency boots, and restoring.

There’s still plenty of learning going on (even among experienced Mac enthusiasts, just see the many posts Howard has made on the topic and the lively discussions below those) and a lot of experiences made along the way that we can exchange. One day, we’ll hopefully have figured out the best practices and have come up with the most efficient ways to deal with all this.

And likely, by then, Apple will have once again made aggressive changes in order to improve system security and we will have to start this relearning process once again. But for us enthusiasts it’s interesting and fun (at times at least). :wink: And despite bumps along the way, it is undoubtedly only getting better as we move farther down this road. :slight_smile:

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Sorry @nello, I certainly didn’t mean to imply you didn’t do your due diligence. :slight_smile: I think the topic was different enough from yours (at least in the way it was worded) that it probably wouldn’t have come up in a search.

My understanding is that if you have an existing TM back drive started in Catalina or earlier, which is formatted HFS+, you can carry on using it, but if you start a new TM backup from Monterey or Big Sur, then TM will reformat (“preparing”) the drive to APFS case sensitive… no choice except to encrypt.

The part in the above above continuing to use a pre-existing HFS TM back up was true for Big Sur, not 100% about Monterey.

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Yes, thank you for pointing out that Monterey apparently requires APFS HDDs to be case-sensitive.

I got a new HDD, formatted it as APFS (not case-sensitive, not encrypted) using Disk Utility and when I selected the HDD as a TM drive, the disk formatting automatically changed to APFS (case-sensitive, and presumably remained not encrypted). I re-formatted it as APFS (not case-sensitive, not encrypted) several times and watched Disk Utility as I added it as a TM drive; each and every time, it automatically converted to “APFS Volume • APFS (Case-sensitive)”

It seems very peculiar to me; Disk Utility didn’t even show the HDD going offline while doing the conversion.

Bottom line: I guess that Monterey TM requires APFS HDDs to be formatted as case-sensitive.

Yes. It uses case sensitivity for TM. You can (and should) use a non-case sensitive volume for the system and data volumes, but the act of creating a TM volume will delete and re-create its volume as APFS case-sensitive, no matter how it was formatted before designating it as a TM volume.

This is also the case when creating TM volumes on Big Sur (when Time Machine on APFS was introduced).

@mikebhm @Shamino

…Is this correct for TM’s?
i.e. Along with DU converting the drive to being case-sensitive, they also force it to use encryption for TM’s now?

Regardless, what are the pros vs cons in encrypting TM backups or not?

The same as they are for any drive. Concern that they might be stolen and have the data harvested.

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No sorry, I worded that very badly. You are not forced to encrypt TM backups. I should have written “There are no choices, except whether to encrypt or not”

If you want to encrypt you do it with the “encrypt” check box in the TM set up window. I don’t know what happens if you do it in DU before selecting the backup disk. I suspect the “preparing” step would re-erase un-encrypted, unless you had checked the “encrypt” box.

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Sure I guess that was obvious. I suppose a better question would be: why would one not encrypt it?

I can’t think of any reason not to, so wonder why they still make it optional, or at least don’t make it the default. :man_shrugging:

If your Mac is too old to have hardware encryption (a T2 or M1), then encryption can slow down disk access.

If something fails, it can be harder to repair/recover data from an encrypted volume, even if you have the password.

Everybody needs determine for themselves if the improved security is worth it. If this is a laptop or portable drive (which could easily be lost or stolen), then absolutely, you should want to encrypt it.

On the other hand, if it’s attached to a desktop system in your home and is never moved, encryption will only matter if someone breaks in and steals the drive. That may or may not be a legitimate concern, depending on where you live.

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