Some Time Machine research + questions

As some of you know, I recently switched from a 10+ year old MacBook Pro running Mojave to a new M2 that came with Ventura and that I quickly upgraded to Sonoma. I have a trusty 2 TB backup drive (from OWC) that I’ve relied on for about 9 years, and I realized a week or 2 after my upgrade that it had quickly gone from well over a third of the drive capacity remaining to around 15%. So I did a bit of research.

I noticed casually that while Time Machine was running (I run it manually about once/week) it had gone from saving around 1-3 GB of data weekly to somewhere between 8-15 GB/week. I thought I might be able to discern what was going on by recording the sizes of backup folders before and after the upgrade, and what I found was surprising: Finder calculations of the backup folder sizes from the 2 weeks before my upgrade were in the range of 230-250GB. Calculating the sizes for 3 of the 4 weeks after the upgrade gave me numbers around 465-470GB. I didn’t expect this and can only surmise that Finder is telling me the size of all my currently active data rather than how much was added or changed each week. (Correct? Each folder size calculation actually took me an hour or 2.).

So now I have a dilemma: my trusty backup drive is down to about 285 GB of free space out of 2000 GB capacity. I also rely on the same drive for a few other things, e.g. storing large media files and app installers, and I do not have the drive segmented, as I’d never felt a need to do that before. (I have also moved a fair amount of older, less needed data to an older portable drive I’d previously stopped using.) How can I best manage this situation before my only backup drive runs out of space?

I’m curious what insights those of you who’ve been on top of this issue for some time may have to offer.

I have an M2 Macbook Air running Sonoma. My main TM backup is a 4Tb hard disk that is nearly always connected. It backs up every hour or so. What I have noticed, compared with my previous setup (an iMac running Mojave) is that the hard disk chugs away for ages, even if I haven’t been using the computer.
This might mean the hard disk is failing but, on the other hand, I suspect that Sonoma is regularly changing many files that TM then needs to backup.

BTW - I have a couple of SSDs that I also use for TM, which happily alternates between attached drives. I do not rely on just one drive for backups.

If your Time Machine backup contains any mission critical or irreplaceable information, I would use its drive for Time Machine only. Why? To reduce the possibility of non-TM use corrupting or damaging the disk. It’s also good practice to make a second backup of your primary drive on its own disk, using a backup program other than TM.

I maintain backups using Time Machine and Carbon Copy Cloner. The Time Machine drive is always connected. The CCC drive is disconnected most of the time. I do a CCC backup about once a week or before installing an OS update.

I do this for redundancy and to increase the chances of having a clean version of my entire setup in the event of a catastrophic failure or a security breach.

As for free disk space, you might find this article interesting:

ETA: Something I like about CCC is that it is much less opaque than TM. The CCC website has a very comprehensive Help section and knowledge base, the software is updated regularly, and the company founder is very engaged with both the product and its users (for example, he will even post on Mac message boards, see Carbon Copy Cloner restore limitations and Carbon Copy Cloner 6 Problems | MacRumors Forums )


If I remember correctly, TM will not delete snapshots corresponding to other computers’ backups. So if you moved a TM drive from your old computer to the new one, you will have a lot of backups that will never get deleted unless you manually do it.

I would suggest you do one of the following:

  • Get a new TM drive, relegating the old one for archival storage.
  • Wipe the TM drive, or delete (most of) the backups from the old computer, after you’re sure the new system is working well and you won’t need to re-migrate any content.

I think the first option is preferable, because a new drive will have a longer life (since you’re not starting with 9 years of usage), and you may also be able to get a larger drive than before. (FWIW, my TM backups are currently being saved to a 4TB HDD).

I’ve never been completely certain about where Apple gets these numbers. It’s been a long time since I read about the details of how TM works, but the last time I looked, the system tracks the directories where data has changed, but then during the backup, TM needs to check each of the files in those directories to know specifically what files have changed.

So does this 8-15 GB mean it has inspected that much data? Or does it mean that that much data has actually changed and is actually transferred during the backup? I just don’t know.

I personally consider that a bad idea. I’d keep these files on a different storage device. I don’t like the idea of merging long-term storage (archive) material with hourly backup/snapshot data. Because you will periodically want to replace or wipe your backup/snapshot storage, but you obviously don’t want to do that to your archives.

I suspect it is touching a lot of files. How much of that data is actually changed would require much more analysis.

WRT performance reflecting failure, that’s always an option, but one thing I noticed several years ago is that large HDDs tend to get hot, especially when powered on 24x7 and used on a regular basis (as is the case for TM). External drive enclosures that don’t have cooling fans may result in overheating, which will absolutely destroy performance and may cause all kinds of weird failures. If your external drive enclosure doesn’t have a fan, consider getting one that does, or get a small fan to blow air across the drive.

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9 years!? . . . Yikes! Time to spend some money . . . :slight_smile:

You should get 3 drives. One for Time Machine; one for a Carbon Copy Cloner weekly backup; and one for your archives. They don’t have to be fast, just sturdy. Something like these OWC drives:
Mercury Elite Pro


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It is, indeed, your second sentence. I, too, have hard drive backup and have noticed particularly since Ventura a lot of disk activity. I poked around a little bit and came to the conclusion that between log writing (you won’t believe how fast log entries scroll in the console) and background processes—iCloud and the like—your disks are being hit far more than they were 5 years ago and for “legitimate” reasons. My old-time programmer self is mildly offended by this profligacy but I’m guessing with all the new Macs using solid state drives they figure “Why not?”


I’ve also seen increased activity on my Time Machine disk more regularly over the last couple of macOS major releases when I’d think it should be idle.

One thing that I’ve found is some interesting behavior if you’ve rebooted your system or dismounted the Time Machine disk. If you remount that disk, macOS will not only mount it, but also mount all available snapshots on a Time Machine disk. I assume that this is in preparation for the possibility that you might want to restore a file using the Time Machine UI. (drop to a shell prompt and do a ‘df -k’ to see what I mean).

For a disk containing a number of Time Machine backups, that will take a while and generate a lot of activity on the backup disk.

I also suspect that there’s some Spotlight indexing occuring on backup snapshots as well.

I’ve read the various replies on this thread, which I appreciate, but I’m still confused. I also read the recommended article on the ambiguities that now come with estimates of free disk space (Explainer: Disk free space – The Eclectic Light Company).

Last Thursday I had an even more jarring experience during my weekly Time Machine backup. After a few weeks of more manageable-sized backups, all well under 10 GB, this one apparently took up over 100 GB of drive space, nearly exhausting the remaining capacity of my backup drive in one session. I hadn’t done any major system updates nor added new apps, though I did recently restart my Mac, which I hadn’t done for a couple of weeks. Maybe I should also mention that I typically keep my backup drive switched off between uses, as the cable adapter I use tends to slip out on occasions, leading to a ’not ejected properly’ error message, which I’m aiming to avoid.

So now my 2 TB backup drive that recently had almost 15% of its capacity left now has substantially <10%, which I know is problematic. And if I’m going to experience 100GB backups on even a semi-regular basis, even a 3X larger replacement drive will max out quickly.

What to do now??

On one side of the coin, old backups will be thinned by Time Machine if it needs the space. The other side of that coin is that the oldest backups will fall off backup disk sooner than they normally would when large amounts of changes are made.

Your problem is boiling down to: what caused that 100GB of backups. A reboot won’t typically generate that amount of data, but other things will.

You say that you keep your backup drive off. That’s going to accumulate hourly backup snapshots on your local drive until you reconnect your backup disk. They aren’t going to thin to 1 days’ worth of local snapshots until they are flushed to the Time Machine drive. Depending on how much data you changed during the time that the drive was disconnected, that might explain a large amount of additional space - all the accumulated snapshots being flushed to the backup disk.

A utility such as BackupLoupe might help you determine exactly what changed in each backup.

A question for you: How much data is used on your Mac, and what’s the oldest backup on your Time Machine drive?

Does Time Machine record ’snapshots’ even if it’s set to only back up manually, as I’ve typically kept it?

Total data on my Mac is only about 290GB. I’ll look up the oldest Time Machine backup when I’m back at my desk tomorrow where the backup drive is located, but I think it’s well over a decade old.

Question: If I start fresh with a new backup drive, can I still access older files from my old drive via Time Machine if there turns out to be something I need that got altered or lost?

If Time Machine automatic backups are off, then local backup snapshots should not be taken.

A decade’s worth of Time Machine backups on a single backup volume is asking for trouble. That means that the backup disk is formatted as HFS+, and HFS+ volumes are known to be brittle.

You should be able to access the old Time Machine volume if you start a new one. But here’s where using a backup solution for long term storage has problems.

  • How do you find something out of 10 years worth of backups?
  • How do you know whatever you backed up can be opened by today’s software?
  • How do you keep something or someone from deleting the files you backed up? Not only ransomware, but the loose nut holding the wheel?
  • How do you protect against that 10 year old spinning rust suddenly deciding it’s no longer going to work?
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I just checked and the earliest folder on my backup drive is from 2015. Clearly I don’t expect to recover anything from that long ago, but without being clear that I can access older files (even a few months old) if I start over, I’ve always just added to the existing backup thread. Thanks for clarifying that.

It’s also clear that I need a new backup drive, as you and David C. have both pointed out. But in the interim, with only 157GB out of 2 TB still available on my backup drive, I’m puzzled about whether I can hang on for another few weeks (until I actually have time for it) or not. How would a 1 week backup suddenly take up over 100GB of disk space when I only have 300GB of data on my Mac and my usual backups are <10GB (based on the progress indicator during my backups)? David suggests that perhaps it’s clocking data it’s inspecting vs. actually backing up, but after last week’s backup I know I actually lost >100 GB of backup disk space.

As a decades-long Mac user, I’m continually perplexed by how much I don’t know about the ways the MacOS has changed in recent years…


I, too, have a TM on a wired external HD (slow, but OK). When it seemed to take much longer than usual, and I believe space was taken up much more than usual, I ran TechTool Pro’s Volume Rebuild routine, not knowing that it’d help and hoping it’d do no damage. It took a VERY LONG (many hours!) to finish, but afterwards it worked as usual again and I got a lot of space back. Might be worth a try. I also run that routine every now and then on my Mac.

My backup today initially looked like it would be <6 GB, but ended up a little over 8 I think.

I’m hoping to gain some understanding of all this odd behavior mainly to be sure my next backup drive will be large enough to handle whatever arises. Or is it likely the inconsistent behavior is mostly from using an older HFS+ formatted drive?

Is that available online or from the App Store?

I bought TTP directly from its maker, Micromat, years ago, and have continually updated it. It’s not cheap, and many feel it’s overpriced, but it once saved my digital life, and I believe it has warded off whatever problems may have come up since then. For me, it’s a must-have.

If Time Machine works as designed, you do not have to worry about running out of space. Time Machine will delete the oldest backups on your drive should it need space to complete a backup.

I would leave that backup drive alone until you get a replacement if you value anything on the drive. Nothing you’ve stated indicates that there’s a problem with the drive that caused your 100+GB anomalous backup. Before you mess around with the drive, you need to perform additional analysis of what went on - either by using something like Howard Oakley’s The Time Machine Mechanic or using a utility such as BackupLoupe (that scans the adjacent backups to figure out what took the 100GB worth of storage). Looking at logs may not be useful depending on how long ago the backup anomaly happened.

In my experience in the data protection world, a backup drive should not be touched except to write new backups, restore backups, or purge old backups. Doing otherwise is playing with fire. Unless you have a copy of the data on that backup drive, you are not far away from making it unusable should you run into “funny things” at the raw disk level.

As I posted earlier:

Don’t hesitate to add a second TM backup drive to your system. Once configured as a TM drive it will be used for backups every second TM session.