In macOS Catalina, your startup disk has a secret. When looking at a hard drive in the Finder, a single icon to represent that volume. In Catalina, your startup disk is now comprised of two volumes, but will still show up as one when viewed from the Finder. Under the hood, the system now resides on a read-only volume, while your data is stored on a separate volume. Catalina uses some tricks to present the two as one whole volume.

When using Techtool Pro 12 with Catalina, the Volume Structures test or Volume Rebuild tool will list both the system and data partitions. The read-only system volume will be the name of your startup disk, while the data volume will be the name of the startup disk with the word Data added to the end of the volume name.

As with previous versions of Techtool Pro, repairing your startup disk – both parts of it – should be performed from a separate volume such as an eDrive or a Protogo device. Testing both of these volumes while using your main startup disk will ‘freeze’ the system during testing.

Pro Tip: If you prefer not to freeze your startup volume during Check Computer’s volume structures test, set up a new Check Computer Suite with your startup volume excluded, or perform that test from an eDrive or Protogo device.