Why can't I upgrade to macOS High Sierra?

Episode 1477 (2:16:09)

Bob from Danville, CA
macOS High Sierra on MacBook Pro

Bob has a 2011 MacBook Pro with an SSD. Now he's trying to upgrade to macOS High Sierra and he's having issues. Leo says that it's looking for the original drive, and since Bob installed it as a secondary drive, it keeps looking to install on the first drive. The simple solution is to swap his drives and put the SSD as the main drive, and the other drive as his second hard drive.