Limit Time Machine disk usage on external drives

Time Machine is a very simple and elegant backup solution for Mac OS, with an intuitive restore browser. The problem with Time Machine however, is that it takes up all free disk space before starting to erase old backups. This is no problem if you have a dedicated Time Machine disk, but most people usually want to keep other things on the very same disk.

Time Machine uses different methods for network backup and local backup. One way of limiting remote backups is covered in an earlier article called “Create a fixed size network storage for Time Machine“, so this will instead focus on limiting the disk usage on locally connected disks, such as USB or Firewire.

First make sure that you are using a HFS formatted disk, since we are going to resize the partition. Start Disk Utility and select your external disk from the left menu. Click the Partitions tab and you will be presented with your entire disk. Drag the bottom-right handle of the partition up and make it as small as you want your Time Machine to be. When you are satisfied with the new size, click the plus button at the bottom to add an additional partition to occupy the free space.

Disk Utility

Now open up the Time Machine preferences and select your disk!

Time Machine Preferences

The additional volume can be used to store anything you want. Just remember to eject the disk properly before you disconnect it from your Mac!