Time-Machine Back-up Size

Tech Support
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I have an 8TB harddrive on M1 MBP (running Sequoia 15.3) my has about 2.36 TB of data. My Time Machine says my backup size is 6.71 TB. This seems out of proportion (?).

I did disconnect my NAS (synology DS1522+) as I’ve just added it and was having failed backups due to the amount of data. I did thin out the snapshot and started with a fresh TM back up I thought(?).

I also deselected some other external drives from the TM options. Is the size of the backup 6.71 TB for 2.36 TB data on main hard drive? Is there a way for me to reduce the size of the backup?

Please excuse if this is a repeat question on here. Thanks for any input

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Time Machine is designed to fill the space it’s given to keep older versions of your data. If you’re running it on a NAS, there are probably settings to cap the space it will take. As you’re setting it back up it may be confused about the space it wants, or be counting older snapshots, which you can trim.

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Thanks …my TM is backing up to an 8TB external drive. I’ve taken the NAS out of the equation for the moment as it seems unnecessary to have (what I guess (?)) is a number of snapshots. I just would like to reset the TM (I think).

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Thanks KB Synology I will look into that … I’m hoping to start with a fresh snapshot of my mac.

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On a Synology, you can limit the amount of space a Time Machine backup will occupy by

  1. creating a dedicated network share for Time Machine backups (which you probably already have, as it’s required);

  2. creating a Time Machine user on the Synology NAS and using that for backups (not your regular user account you use to connect to the NAS!), and;

  3. setting the quota for the Time Machine user (for example, if the “Time Machine User” has a quota of 4 TB, and you use that username for Time Machine, it won’t be able to go over 4 TB). Once it approaches that limit, it will start deleting the oldest backups.

Otherwise, when using external drives, Time Machine is designed to fill up the available space by keeping backups further from the past, which enables you to recover a file you possibly had but lost from weeks or months ago, or to restore an earlier version of it.

You can limit the size of Time Machine backups on external drives by partitioning the drive and using just one of the partitions for Time Machine, although Apple recommends using a separate, dedicated drive for backups (which is a sensible approach).

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What @dario explains is exactly what I do for TimeMachine backups to Synology NAS. I set the allocation as double the size of the machine being backed-up.

Synology explains all this at:

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Just to add that, yes, it does make sense to have two Time Machine backup destinations (for example, a NAS and an external drive).

Time Machine backups can sometimes become broken (which is when Time Machine wants to start from scratch, and everything is lost), and backups stored on network shares seem particularly susceptible to this. This will happen every once in a while, at least for me. Time Machine backups stored on a network share seem semi-officially supported by Apple ever since they stopped selling and supporting their Time Capsules. While they are more convenient if you own a NAS, I would not rely on them alone for any work-critical backups.

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Another related question (or wishful thinking…(?)) I’m going to be setting up a NAS; if I set a User and limit the amount of back-up to 5 TB will it automatically recalibrate the backups to adjust to the new size?

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I’ve resolved this issue … I read the logic of the options backward. I didn’t realise that I had to add the drive saying exclude. Harsh learning experience. Thanks all!!!