Hi J: If you opt to work from an external USB3 drive, you definitely want to review the link that Meg The Dog pasted into her response. With that workflow, you can edit directly from your USB3 drive and export High Resolution edited masters to your internal Fusion drive (of course, “Fusion Drive” is just Apple’s marketing department’s name for “hybrid drive”). If your budget allows for it, purchase an external Thunderbolt RAID0. Depending on the bandwidth requirements of your 4K footage, you might be able to use a non-RAID external Thunderbolt drive instead. For your model iMac, Thunderbolt2 is a match; however, you should be fine with a Thunderbolt3 device as long as you have the adapter (without the Thunderbolt3 to 2 adapter, you won't be able to connect the device). Promise, LaCie and G-Tech are each popular drive makers, but definitely shop around for the best capacity, speed, warranty and price. -Warren
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