When I try to run my app with Xcode 6 Beta 4 on the Simulator iOS 8.0 I always get this error:
Unable to boot the iOS Simulator
Even I encounter same problem, after a bit of toying around with Xcode and iOS simulator I am able to fix this issue with following steps:
First I will try to Reset Content and Settings... in OS Simulator.
If it doesn't work I will quit iOS Simulator and try.
Still if it doesn't work I will quit both Xcode and iOS Simulator.
You should be able to fix things using above steps, if it doesn't, try reboot and then see if you can get things up and running.