Time Machine normally keeps backing up all versions of the files until the backup drive is full, then it automatically deletes the oldest backups. It’s a good idea to use a backup drive 25% larger than the capacity of the source drive(s) so Time Machine can keep older files around in case they are needed later. If the source drive has a 500 GB capacity, a 700 GB backup drive is plenty.
When you have several Macs, you don’t want to keep buying hard drives. You can attach the same drive to several Macs, backing them up one at a time. Better yet, share a drive with an AirPort Extreme, Time Capsule or OS X Server for automatic, simultaneously network backups. You just need to have a plan to balance the load.
Suppose you have a 1 TB backup drive and you back up a Mac with 250 GB of files. A couple years go by and now the Mac has 350 GB of files. But that’s only part of the story. Some files have been changed, some deleted, and new files added. The backup has all the current files, plus all the files modified and deleted over the years, and it may have grown to 600 GB. Now suppose you want to add a new computer with 350 GB of files to the backup. Although it will fit alongside the first Mac, the new computer will be at a disadvantage.
Time Machine will fill the space and delete old backups as usual. But each Mac’s backups are considered independently. When Time Machine deletes old backups from the first Mac they are several years old. But the new Mac’s “old backups” might be from just a few weeks ago. If you need a file from 6 months back on the newer Mac, you might be out of luck.
You could exclude files from the backup to reduce the size of future backups, but that doesn’t have any effect on the existing backup size. You can manually delete files from the backup to regain space, but that’s a lot of work and may not avoid the problem in the future. What if you could set a maximum size for each computer’s backup? You can.
Setting a maximum Time Machine size
- Go to the computer where you’d like to set a limit.
- Log in as an admin user and open Terminal.
- Type or copy & paste the following command: defaults write com.apple.TimeMachine MaxSize and a space.
- Type the number of megabytes for the limit. To roughly convert from gigabytes, add three zeros. Then hit return.
- Want to remove the restriction? Type defaults delete com.apple.TimeMachine MaxSize and hit return.
If you set the limit lower than the current backup size the next backup may take longer as it removes old backups to achieve the new limit. Setting a limit reserves extra room on the drive but we don’t recommend using this space for non-backup files. If you store original copies of important files on the backup drive, they may not be backed up anywhere. That could result in catastrophic loss if the backup drive fails.
Finally, OS X allows a Mac to back up to several Time Machine destinations. When you set a maximum backup size you’re setting the same limit for all destinations. If you get a bigger backup drive you may want to change the maximum Time Machine size.
Balancing network backups
When several Macs share a network backup volume (on a server or Time Capsule), Time Machine creates a disk image for each one. It automatically sets the maximum size of each disk image to the total capacity of the backup volume – in effect, allowing the backups to fight it out for themselves. As described earlier, this can cause issues.
OS X Server lets you to limit the size of each backup in the Time Machine service. But this only works when both the server and workstations use OS X Mavericks or later, and even then all computers are subject to the same limit. With two Unix commands you can apply individual limits on OS X Server, Time Capsule or other network backup drives.
Limiting the backup disk image size
Already started backups? Make sure either Time Machine or the computer is turned off, then skip to step 4.
- Set a Mac to use the network drive as the Time Machine destination and Back Up Now.
- Wait 5 minutes for Time Machine to create the backup disk image on the shared volume.
- Turn off Time Machine. Wait another 5 minutes for Time Machine to stop the backup.
- Log in as an admin user on any Mac that has write privileges to the network backup drive.
- Access the network drive. Do not open the disk image.
- Open Terminal and type or copy & paste the following command hdiutil resize -size 500g and a space. Substitute your own desired size (in gigabytes) before the g.
- Drag the disk image for the computer to the terminal window and hit return. Wait for the operation to complete.
- Type or copy & paste sudo chflags uchg and a space. Drag the disk image to the terminal window. Then click the terminal window and type the delete key to remove a space. Type /Info.* (no spaces) and hit return. Type the admin password. This tells Time Machine to keep the limit - otherwise Time Machine changes it back to the default at the next backup.
- Turn on Time Machine.
If you’ve already backed up a computer you cannot set the limit lower than the current size of its backup disk image. If this is required, you will need to first reduce the backup size with the time machine limit (above) or delete files from the backup. Then you can limit the backup disk image size.
Sample case: You have a 2 TB Time Capsule and 6 laptops, each with a 250 GB drive. Limit the backup disk images to 333 GB and each laptop has its own reserved space without taking more than its fair share.
Manage Time Machine when you replace hard drives or computers.
