I just bought a 5K iMac with 4G i7 & 512G SSD. I thought about buying the 3T fusion, but figure 3T is not big enough anyway. So if I bought the 3T fusion, I have a 128G internal SSD with a internal 3T spinner. I'd still need to add a external drive anyway. So I bought with the 512G internal SSD. So ... I could buy a 8T external spinner and either 2 "drives" or have an 8.5T "Fusion" drive. What are the pros & cons to these setups? Are there better ideas?