What about my Time Machine disk?

Time Machine drives present an interesting challenge when it comes to drive maintenance. This is primarily because they store many versions of each of your files as they have changed over time. In the volume structures of the disk, this is achieved using what is known as a hard link. Rather than store copies of each file with each backup, links are created to tell the disk to store multiple references to the data that makes up the original file, and only store new data when a file changes. While this saves lots of disk space, it makes the volume structures of Time Machine disks especially complicated.

Time Machine Color Burst

The unusual characteristics of Time Machine disks make performing maintenance on these disks especially challenging. Running the File Structures test, for instance, can take an especially long time, because it must follow each link, meaning that it ends up checking the same raw data multiple times. And when a Volume Rebuild is run, the sheer size of the volume structures on a Time Machine can present special challenges.

So with a Time Machine, it turns out that less is more. If you're regularly running the File Structures test on the disk that is backed up to Time Machine, you've essentially already run the File Structures test on the Time Machine's most recent backup, which is what you're *most likely* to restore from. And using the Volume Rebuild tool to optimize the Volume Structures can be an exercise in frustration, as the volume structures tend to get pretty fragmented after each backup. So we recommend you skip the File Structures test for a Time Machine disk, and use the Volume Rebuild exclusively when repairs are necessary.