Time Machine is an excellent backup tool for any Mac. However, by default, it keeps all backup versions until the backup destination, be it an external drive or a storage server, is full.
This practice can be wasteful, and eventually, you'll have no space for other uses, like storing files. It's especially problematic when you use a backup server. Since one Mac can hog all the storage space, you'll end up having nothing left for other computers and services.
That said, it's a good idea to limit the amount of storage Time Machine can use.
Determine the OS version and suitable backup limit
Before you can fix Time Machine's storage, you first need to determine what version of macOS you're running.
(If you don't know how, check out this post, which also gives you some other handy Mac skills, including how to run Terminal, which you'll need to use in this post.)
After that, find out the amount of storage you need for the backups. Generally, you want to use 150% or more of your Mac's internal drive capacity.
So, for example, if your MacBook has 128GB of internal storage, 300GB of backup space is generous. But pick a number that suits your needs. The more storage space, the more versions of backups you'll get, which means the further you can go back in time.
Limit Time Machine backup size in macOS 10.12 and earlier
This trick is easy to implement and for sure works on macOS 10.12 (Sierra) and older versions. Note that you need to log in with an administrator account.
Here are the steps:
- Convert the storage limited you want into megabytes. So if you're going to use 300GB, then the number you have is 307200 since 1GB = 1,024MB.
- Run Terminal (you can search for it using Spotlight at the top right corner). Type in this command, then press Enter:
sudo defaults write /Library/Preferences/com.apple.TimeMachine MaxSize 307200
You'll be likely prompted to enter your account's password, so do that and press Enter again. And that's it! From now on, Time Machine on that Mac will automatically delete old backups to make sure it uses no more than 300GB.
If you want to revert to the default value (no limit), run this command:
sudo defaults delete /Library/Preferences/com.apple.TimeMachine MaxSize
Limit Time Machine backup size in all versions of macOS
If you run the latest macOS (10.13 Mojave or newer), the command above won't take effect. Instead, you'll need to limit the storage space at the destination side, be it the external drive you plug directly into your Mac or a Time Machine-supported NAS server.
By the way, since you make changes to the backup destination, this method will work on all macOS versions. But it does take a bit more work.
Limit Time Machine backup size of an external drive
The idea here is to create a separate partition on the external drive and use that partition exclusively for Time Machine backup. For that, you turn a single hardware drive into two different volumes (or logical disks) and use one for Time Machine.
Note: Doing this will erase all drive content, so make sure you're OK with that.
Here are the steps:
1. Connect the external drive you want to use for Time Machine to your Mac and call up Disk Utility (you can find it using Spotlight). In Disk Utility, select the drive, then click on Partition.
2. Click on the Plus (+) to add another partition to the drive, then pick the Name, Format, and Size for the new partition. (For this example, I named the new partition Time Machine, used Mac OS Extended (Journaled) as the format and 300GB as its size.) Then click on Apply.
By the way, if you're curious as to what file format to use. Here's the breakdown:
- APFS only works with Sierra (10.12) or later.
- Case sensitive means files of the same name but different letter cases (i.e., MyFile.txt vs. myfile.txt) exist as two separate files.
- Encrypted means data stored on the drive can be encrypted, use this if you intend to store sensitive data on the drive. The catch is if you forget the password, you'll lose the data.
- macOS Extended Journaled (a.k.a HFS+) works with all Mac OS versions and will not work with Windows.
- exFAT works with both Windows and Mac but does not support Time Machine backup.
- MS-DOS (FAT) is an ancient file system, and generally, you shouldn’t use it.
3. Confirm the action by clicking on Partition. The drive will then have two partitions. One of them is 300GB, and the other has the rest of the drive's storage space.
4. Run Time Machine (again, you can use Spotlight to call it up) and use the newly created partition as the backup destination.
And that's it! You can now use the other partition for file storage; Time Machine will never touch it. By the way, if you use a Time Machine-supported router, such as the Asus RT-AC86U, this method works, too.
Limit Time Machine backup size on a NAS server
This part depends on the NAS you use. Similarly, you can create a separate volume and use it exclusively for Time Machine. Some servers also allow for setting a storage limit (quota) for an existing shared folder.
If you happen to use a Synology NAS server, like the DS1618+ or the DS1019+, which I would recommend over any other NAS brand, you can easily do this by editing a shared folder within the Control Panel.
Remember that the folder share quota is only available when you use the NAS with the btrfs file system.
Dong's note: This post was originally published on Mar 21, 2018, and has been updated.