Starting with macOS 10.13 ‘High Sierra’, certain low-level operations became unavailable to applications like TechTool Pro. One of these limitations is the inability to read the raw disk of the physical drive containing the startup volume. As a result, we cannot perform a Surface Scan on a drive if it is being used as the boot volume.

To address this, you will need to start up from a separate physical drive to run the Surface Scan test on your normal boot volume. If you are using macOS 11 ‘Big Sur’ or earlier, this can be achieved by creating a TechTool Protogo device on an external drive, such as a USB flash drive, or by building your eDrive on an external drive.

If you’re using macOS 12 ‘Monterey’ or later, you’ll need to download the macOS Installer application from the Apple App Store, then run the installer application and select an external drive as the destination volume to make it bootable. After installing the operating system on that volume, boot from it to ensure it starts your Mac, and then install TechTool Pro. While booted from this newly created boot volume and with Techtool Pro installed on it, you will be able to run a Surface Scan on the internal drive that contains your normal boot volume.