I am deciding between ordering just SDD or SDD + HDD. My concern is- will HDD will introduce noise to otherwise silent SSD-only iMac ? Since OS is installed on SSD, does second HDD spin down when it's idle? If HDD spin down does it create that annoying problem when every time you open File Open/Save dialog system hangs/pauses for a few seconds waiting for 2nd drive to wake up? Can you tell me about your experience having 2nd HDD in iMac regarding to noise and general usefulness. Or I am better off just having external drive.