I’ve gotten this message “Unable to determine simulator device to boot.” a few times when trying to run my app in the iOS Simulator. 
Each time it took me awhile to remember what it meant.
I’ve been working on updating an app to iOS8 and using Xcode 6. Occasionally I need to run the current version and I start up Xcode 5. When trying to run the app in the simulator I see this error.
The reason for the error is because the Xcode 6 simulator is already running and it doesn’t know which one to use. 
The solution is simple. Just quit both simulators and try again run again.
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