What Dan said is correct, you can install 2.5” drives (with an adapter like this one), 3.5” drives, or strange amalgam PCIe/NVME/SATA cards such as this one and this one. Now, depending on how much storage you want, how fast you want it to be, and how much your budget is, there are many options. Like Dan said, we need to know what kind of drive you want, and we’re be happy to help you pick one.
If you’d prefer to choose a drive on your own, I’d recommend using Disk Prices and PCPartPicker (I set up the links to only show compatible drives).
According to Apple, you can install fibre channel cards, fiber networking cards, video and audio I/O cards, storage cards, and Ethernet cards.
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