Did you know that Techtool Pro can test and repair your startup disk without a separate startup volume? Starting with Techtool Pro 14, however, the Volume Structures test and Volume Rebuild tool provide live testing and repair of your APFS startup volume, no extra startup disk is required. While a separate startup volume can still be useful, macOS Monterey no longer allows third parties to build secondary startup disks. However, you can still make your own.
It is important to know that a disk cannot change during a test or repair. As a result, the system will ‘freeze’ during tests and repairs. In certain circumstances, this freeze can take a while, and it doesn’t mean that something’s wrong, even though an unresponsive Mac can certainly feel unnerving. If you have a few APFS snapshots lying around (you can check with the Snapshot Recovery tool), this process can be especially long. You may even want to use the Snapshot Recovery tool to pare down the number of snapshots before testing your startup disk.
Pro Tip: As always, make sure that your data is backed up, particularly when performing repairs, because not all volume corruption can be fixed.