Techtool Pro provides a number of ways to create a secondary startup disk for troubleshooting purposes. The eDrive and Protogo tools provide ways to either add a startup partition to an existing disk or create a dedicated drive, respectively. Whether you’ve created an eDrive or Protogo device, when you’re done using it, you (most likely) will want to return to your primary startup disk.
The Startup Disk system preference pane is perhaps the most familiar way for assigning a startup disk for your Mac. It’s certainly the most straightforward. Open the Startup Disk system preference pane, select a startup disk and you’re done. It may not be the best method, however, when you want to start up from a different startup disk only once. The reason for this is that when you select a startup disk in System Preferences, you’re telling your Mac that you always want to use that disk to be your startup disk.
The less well-known, but probably better, method for selecting your eDrive or Protogo is to hold the Option key at startup. Holding the Option key will tell your Mac to pause startup and bring up the Startup Manager, allowing you to choose a startup disk. You can even use the arrow keys on your keyboard to switch between disks if your mouse or trackpad hasn’t connected yet. Even if there are no startup disks, it will stop and wait for one to appear. The best part about this method of choosing a startup disk is that it does not change the startup disk in your Mac’s preferences, so when you restart, your Mac will return to the previous startup disk.
There is an additional consdieration if your eDrive or Protogo device has not been updated in a while, and your primary startup disk is running High Sierra (macOS 10.13). Because the eDrive and Protogo devices must live within the rules of the macOS version installed on them, they may not even see a High Sierra or Mojave startup disk from their Startup Disk system preference pane. The reason for this is that older versions of macOS don’t support APFS. Because of this, they don’t recognize the existence of newer macOS versions, which are installed on volumes formatted APFS.
Our recommendation is to use the Option key startup method when possible to start up from your eDrive or Protogo device.
Pro Tip: If the Option key isn’t registering at startup, try a wired keyboard connected directly to your Mac.
BONUS Pro Tip: If you have a T2 equipped Mac, here’s important information about using eDrive and Protogo.