I have 4 drives connected to my iMac, one of which is Time Machine backup drive. Every time I try to boot with the external drives connected to the machine, it stays in gray booting screen. It doesn't stop or halt, I simply need to disconnect or turn off my drive docks. I certainly checked startup disk setting, it was set to Mac. Has anyone encountered this kind of problem before? I never had such problem before, so I'm not sure if this is ordinary behavior.