What are Volume Structures?

Now that most of us have GPS enabled devices in our pockets, it is (mostly) straightforward to find our way when we're out on the road. When it comes to your hard drive, macOS needs its own GPS to find where your files are located when you need them. This GPS, or map, is how you can think of your drive's volume structures. When the map is damaged, your system loses its way and may be unable to navigate to your important files.

Catalina Drive Map

For most Mac drives, volume structures come in one of two "flavors," Mac OS Extended or APFS. If you are using macOS High Sierra or later, your startup drive uses the newer APFS. While APFS adds new features to better support the solid state drives used in almost all modern Macs, Mac OS Extended can still be found on many external storage drives.

Whereas mechanical failures such as bad blocks or SMART errors most often require that a drive be replaced, volume structures problems can often be repaired. In essence, a volume structures repair is akin to drawing in the missing pieces in the map, provided there is enough data to determine what is missing. The Volume Structures test in Techtool Pro can determine if there are holes in the map, while the Volume Rebuild tool is for repairing the map. A note here that repairing your startup disk requires starting up from an alternate disk. Here is more information on using either an eDrive or Protogo device to perform volume structures repairs.

Pro Tip: We can't say it enough - make sure your back up is always up to date. See our previous pro tip for backup tips.