If you've been using an eDrive or Protogo device and find that your startup disk isn't available in the Startup Disk preference pane to return to your primary startup disk, it may be that the version of macOS on your eDrive or Protogo device is out of date.

In this situtation, you can hold the Option key at startup to bring up the Startup Manager, which is (typically) a gray screen with any available startup disks displayed. (Note: it should still come up if there are *no* startup disks available. If not, there may be a communication issue with your keyboard.) From the Startup Manager, you can use the arrow keys to choose which startup disk to use for the next start up. 

If you have chosen your eDrive or Protogo device from the Startup Disk system preference pane, set it back to your primary startup disk. From your primary startup disk, open System Preferences and choose the Startup Disk preference pane. After clicking the lock icon so that it appears unlocked, single click your primary startup disk so that it is highlighted.