How does Time Machine work with limited external drive space and iMac memory?

Zenon Stoikos wrote:

I have been backing up for some years my iMac (1TB memory) onto an external drive (1TB).

The external drive now shows 323GB of 1TB available. Time Machine shows backups from 4 years past.

(a) Does this mean that I can still continue backing up ...

Yes. The subsequent clause "until the older backups come closer to recent" is not a factor. Older backups become deleted automatically as Time Machine replaces them with newer backups, when TM requires the space they occupy.

On that subject if you intend to use a significant percentage of that Mac's 1 TB startup drive, you will find a 1 TB backup drive to be insufficient. Apple has traditionally recommended using a backup drive of at least twice the Mac's internal capacity, and some references suggest 2x to 3x its capacity. As with most "rules of thumb" that one is overly broad and lacks strict justification, but if you follow that advice it will keep you out of trouble.

It's up to you to determine if you really need an item that was backed up four years ago, or ten, or twenty. Yes TM has been around nearly that long.

(b) Where in iMac can I see how much of memory has actually been used?

Memory is not a factor. Settings > Time Machine will show the backup date ranges.

(c) If it is time to replace the external drive, what happens with the current iMac memory? Do I delete it somehow? And retrieve from the external drive?

Memory is not a factor. Replace the backup drive when it no longer works, becomes unreliable, etc. For that reason it is best to have more than just one backup drive. TM will back up to two separate backup drives, or three, and so forth. Each drive will contain the bare minimum of one fully restorable backup.

If you mean at some point the Mac's startup disk might become full, as in reaching its capacity limit, yes that can happen. If it does, macOS first resorts to a number of increasingly desperate measures to work within its remaining space, but if those efforts fail eventually it will throw a warning that you need to take action. That would be Free up storage space on Mac - Apple Support.

Memory is not storage — separate concepts altogether. When you buy a Mac, you decide how much of each you want. More is always better and more expensive, and it's not possible to have too much of either. 1 TB is fine — "for most people" — but only you can decide how much is enough. If you're producing full length production quality videos (for example) you would want a lot more than 1 TB.

Zenon Stoikos wrote:

Even though 1TB on SSD is quite adequate for my needs, and I may even delete or transfer large files elsewhere externally.

Two things:

1. The advice to use a TM disk that's “2 or 3 times the size of the internal disk” is not strictly accurate. It should be “2 or 3 times the size of WHAT IS BEING USED ON the internal disk”. If, like me, you only use around half of a 1TB drive (and are not expecting to increase this radically), then a TM disk of at least 1TB is adequate. The fact that you've 4 years’ worth of backups on your external with still a third yet to fill says everything to me. You have no need to concern yourself on that point for a while yet.

2. You might find it worth looking to see if any of the large files being backed up should be in TM at all - for example I started noticing that TM was deleting my oldest backup on a daily basis as it was running out of space. On closer investigation I found that TechTool Pro was creating a complete directory listing file 3 times a day, each one 1.5GB, so every day TM was using up 4.5GB of my external drive. When I realised this, I excluded those files from backup then went in and deleted every backup of those files, and got back over 300GB drive space!

So we're all on the same page the 1 TB drive is called storage, not memory. Memory is what you have shown in the About This Mac, usually 8 or 16 GB. Memory is used to open files into and manage them via editing, etc.

I'm sure it will be fine.

The more technically correct answer is that you need a TM backup disk with a capacity sufficient to retain one complete, restorable system backup plus enough excess to create a subsequent backup (which can be quite large) plus an additional amount of "overhead" for workspace that is not easily calculated. Even TM has to resort to little more than an educated guess, which tends to be overly generous because TM will not even initiate a backup until it determines a successful completion is likely. Only after that will TM begin to delete older, "expired" backups while preserving the one it just created.

That's how those "rules of thumb" arise — it precludes Apple having to explain why a backup disk may have hundreds of GB free space remaining, yet TM is complaining it's not enough.

It gets worse, because it has become difficult for us to even determine how much storage a Mac is actually using. The amount can vary by many, many GB, both up and down over the course of simply using a Mac on a daily basis.

christopher rigby1 wrote:

Zenon Stoikos wrote:

Even though 1TB on SSD is quite adequate for my needs, and I may even delete or transfer large files elsewhere externally.

Two things:

The advice to use a TM disk that's “2 or 3 times the size of the internal disk” is not strictly accurate. It should be “2 or 3 times the size of WHAT IS BEING USED ON1. the internal disk”. If, like me, you only use around half of a 1TB drive (and are not expecting to increase this radically), then a TM disk of at least 1TB is adequate. The fact that you've 4 years’ worth of backups on your external with still a third yet to fill says everything to me. You have no need to concern yourself on that point for a while yet.

It’s an aggregate of storage usage and of storage churn; how much data is present, and also how much and how often that data is changing. This for the user’s expected depth of backups, and for the user’s expected churn.

That’s also a higher-risk and higher-effort approach.

For not all that much reward.

Storage usage often increases over time. Incrementally-increasing storage usage often isn’t noticed until a larger macOS update or even a bigger file getting churned derails Time Machine. Which is probably the trigger for this thread, too.

Churn can vary. Dozens of small files churning, for instance, depends on how fast those files are churning. Larger files churning, such as uncompressed virtual machine guests, or such as the results of a macOS update or some big app or game upgrade, can (will) cause issues for Time Machine configured with inadequately-sized external storage.

Now can a tuned archive device storage capacity work? Somebody that picked both main storage and Time Machine archive of the same storage capacity — and that consistently manages their storage usage to allow some depth of backup — may well utilize this, sure.

But those folks (and I include myself here) that don’t want to focus on or don’t or can’t manage their ongoing storage usage then can (do) get in trouble with too-small archive configurations. And this added focus for the price difference (saving) between a one terabyte storage device and a three or four or five terabyte storage device. Which often isn’t all that much. Not when compared with the cost of my focus, and the value of my data.

Much like buying an under-configured iPad (or “under-purchased”, as some folks call that), sure, the storage contents can be managed, off-loaded, re-loaded, and shuffled around as needed and as the workload and focus changes, or the depth of backups can be constrained, or a combination, and this saving some money on the storage is entirely possible. The trade-off is in the cost of managing the resulting configuration when it needs attention, rather than being able to largely ignore it all, and let it self-manage.

2. You might find it worth looking to see if any of the large files being backed up should be in TM at all - for example I started noticing that TM was deleting my oldest backup on a daily basis as it was running out of space. On closer investigation I found that TechTool Pro was creating a complete directory listing file 3 times a day, each one 1.5GB, so every day TM was using up 4.5GB of my external drive. When I realised this, I excluded those files from backup then went in and deleted every backup of those files, and got back over 300GB drive space!

That per-backup usage is roughly 1.5% of a one terabyte storage device, per backup that surviving the usual pruning. It’s closer to 0.5% of an external backup device sized more appropriately for Time Machine of a one terabyte internal storage; for depth of backup and for churn.

Now is it appropriate to exclude that or other similar transient data from a backup? Sure. (I’d also wonder why that tool either didn’t suggest that or didn’t exclude itself, or why it’s not simply using the existing Spotlight data. It’s tool intended for people that want to tinker, so adding settings to tinker with the tinkering tool is appropriate.)

As another consideration for backups, Time Machine doesn’t keep backup copies long-term. As the storage capacity of the external storage is reached, the oldest backups are purged. That involves either additional sets of backups, or rotating Time Machine backups, and long-term archives of separately-focused backups, or of Time Machine backups, or a combination. If you want or need to keep copies of certain files ~permanently, you’ll need to made specific additional arraignments, as well as arraignments for ongoing preservation of access to that older data.

TL;DR: the price difference between a one terabyte external disk and a three or four terabyte external disk isn’t big, particularly given the cost of the Mac, the cost of your time, and particularly the value of your data. How much is that external storage saving worth to you?

John Galt wrote:

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"Churn" is a good description for a phenomenon not easily described.

In SSD terminology, it’s Drive Writes Per Day (DWPD), among other nomenclature.

Sizing suggestions based on internal capacity are a whole lot easier than would be explaining the nuances, and easier than dealing with the support fallout when an undersized external meets an overfull internal.

For middling- and larger configurations, folks are probably sharing a NAS or two, and pooling archive capacity across multiple users.

The bottom line, Mr Hoffman and John Galt, is that the OP reports he has a third of his 1TB external remaining, even with 4 years of TM backups, so I can safely infer that he has no impending concerns, not for a good while yet.

Regarding (c), note that Time Machine can quite happily use more than one external drive for separate backups. It will alternate between the drives when backing up. You don't have to retire the current drive - just add the new drive in Time Machine settings. This introduces redundancy in your backups.

👍

"Churn" is a good description for a phenomenon not easily described.

In the past I recommended the backup drive of sufficient capacity based on the amount of storage actually used, but that is not as easily determined any more.

Apple never changed their recommendation, which was always predicated on the source drive's capacity: If Time Machine on your Mac recommends a larger backup disk - Apple Support. Obviously they knew what was coming.

You're welcome. My philosophy is you can never have too much external storage. Glad you've got your solutions.

It should be noted that Time Machine is always "running out of space" and is always deleting the oldest backups. That is completely normal and expected Time Machine behavior. It is doing what it should.

"Tech Tool" on the other hand is another story.

When I realised this, I excluded those files from backup then went in and deleted every backup of those files, and got back over 300GB drive space!

... which will eventually become occupied, upon which TM will resume deleting the oldest backups, doing exactly what it is designed to do.

The boldface text though implies you may have manually intervened in Time Machine's operation and rendered the entire backup corrupt and possibly useless. You may need to revisit that subject, as well as reconsider the use of garbage products that serve no beneficial purpose.