Huw much local disk space does Time Machine need? (Mojave)

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torsig1967 Author

Level 1

21 points

Huw much local disk space does Time Machine need? (Mojave)

Hi. I hade some issues with Time Machine simply not making backups but no error messages. Same on several har drives. After initial Time Machine preparation, nothing. This was on Mac Book Pro (2016) with Mojave.

Finally I found some advice about freeing up disk space on the source disk (not the disk I'm backing up to). When it was not working I had 15 GB free which I would consider quite enough. But I freed up some more and got 75 GB free (drive aprox 500 GB) and then Time Machine backup worked on all backup drives.

I find this very strange. Does Time Machine really need that much free local space? There's no indication it clamied a lot of disk space during backup (maybe 1 GB at most). Why was 15 GB not enough?

Mac mini, macOS 10.12

Posted on Dec 11, 2023 12:37 AM

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Answer 1

Top-ranking reply

Antonio Rocco

Level 6

12,404 points

Dec 11, 2023 2:40 AM in response to torsig1967

"Why was 15GB not enough"

As a general 'rule of thumb' (for any computer/OS be it PC, Mac or Linux) you should keep at least 10% of the hard drive capacity "free" at all times. This is because some processor and application (Photoshop in particular) based activities are disk-related and any free disk space is used for that purpose. As an example a 250GB hard drive should have at least 25GB of free space and so on. As a reasonably heavy Photoshop user I try to keep at least half of my 500GB SSD drive free at all times.

Time Machine should use a hard drive that is at least double the size of your internal drive's capacity. I would have at least two drives for use - more if you can afford it.

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Answer 2

Dec 11, 2023 5:49 AM in response to torsig1967

Not being a fan of TM all I can think of is it’s probably by design. Maybe someone else may contribute who is more familiar with TM’s internal workings.

In the meantime and if you want to persist with TM, I would offload to one of your other external drives as much data you don’t need continuous access too as a way of pre-empting the problem.

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Answer 3

Dec 11, 2023 7:55 AM in response to torsig1967

Carbon Copy Cloner. Been using it for years on servers as well as personal computers, never fails.

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Answer 4

torsig1967 Author

Level 1

21 points

Dec 11, 2023 2:59 AM in response to Antonio Rocco

Thanks, yes as a rule of thumb I can see your point. I should not use the last 50 GB on my 500 GB drive.

Still, from a technical perspective, I find it very strange that Time Machine quits silently because there's not enough hard drive space when there's still 15 GB free and no indication it is being used by the system.

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Answer 5

torsig1967 Author

Level 1

21 points

Dec 11, 2023 7:16 AM in response to Antonio Rocco

I also use rsync in the Terminal a lot but I find that more appropriate for "important folders" that change often and that I want to keep om special backup disks. But as a backup of the entire system do you have any advice on programs more convenient and reliable than Time Machine? I must say, I've never had any issues with Time Machine prior to this, used it for some ten years, but it might be time for a change.

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Huw much local disk space does Time Machine need? (Mojave)

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