2013 imac in order to chain external hard drives..-seems like a simple request hahhahah--wrong. my option is to bother this forum...or the dreaded apple support, that usually goes with much angst,- wear down two new friendly chipper less informed than I am trainees, then after an hour several dozens screens read to me ver batem over the phone. I get an "advisor" who knows what they are doing are fully trained, not drunk, and somewhat burned out. not anything I look forward to...thanks for you help, guidance or just tell me where to look- plug in my serial # and walla...thank you for your trouble not sure why this is so difficult. apple store is 100 miles away --so part number and will order online..