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Instead of replacing my HD (Fusion drive) with SSD, can I replace the HD with the same size or larger HD?
Sounds like you have the idea a Fusion Drive is one device, which it isn’t! It is really two devices a SATA HDD and a PCIe blade SSD. Each device uses its own port within the system (your system only has two ports). So… Replacing the HDD with another HDD of the same size will not change anything other than if your current HDD has failed. Replacing the HDD drive with something larger will give you the added space (1 TB for a 2TB, a 2TB for a 3TB and so on…) If you are looking for performance replacing out the small blade SSD your system currently has with something larger will get you a big leg up. iMac Intel 27" Retina 5K Display Hard Drive ReplacementiMac Intel 27" Retina 5K Display Blade SSD Replacement As far as to what drives to get if you are just looking at replacing the HDD and re-enable the Fusion Drive set then stick with the same sized drive. The blade SSD Apple uses in a Fusion Drive set is quite small and is often too small to be effective when a larger drive is used. If you are looking for a bigger better performance HDD I would go with a hybrid drive like a Seagate FireCuda SSHD. Don’t bother with the blade SSD in this setup. The last option is leaving the HDD alone and get a bigger blade SSD, a 512 GB will get you a fair amount of performance a 1TB will get you a lot if you are working on video or music production. Stick with either an Apple SSD or OWC. Just make sure its the right one review this great guide: The Ultimate Guide to Apple’s Proprietary SSDs
