Solid-state drives (SSDs) are expensive, especially if want a capacity above 1TB. That's why hard drives still rule the roost, even though they don't offer the speed of an SSD. Apple's software-based Fusion Drive provides a compromise: it uses a small amount of high-performance SSD alongside a higher-capacity HDD. macOS caches frequently used drive-based data in the SSD, boosting performance.
When deciding on a drive for an iMac I purchased earlier this year, I felt that the performance I’d get from the $700 jump from a 1TB Fusion Drive to a 1TB SSD simply wasn’t worth it. Apple pairs a 32GB SSD with its 1TB hard drive, and 128GB with its 2TB and 3TB options.
Macworld reader Terence would like to upgrade an older iMac to a newer version of macOS, and wants to create his own Fusion Drive. Apple’s technical support told him, he says, that he can migrate to High Sierra and then use it to create a Fusion Drive with bring-your-own-drive options. The folks at OWC have a detailed blog post about the technical steps to pull this together. (Terence also wonders about how to set this up given that High Sierra will force APFS on an SSD, but since it doesn’t work with Fusion Drives yet, I don’t think that’s an issue.)
I’d say the far better option, if you’re purchasing new drives anyway, is to find a hybrid drive with a good reputation and go with it. A hybrid drive is a single drive that combines a SSD with a hard drive. These seem to mostly max out at about 8GB of SSD, but can cost under $100 for a 1TB/8GB model. The caching happens below the driver level, so the drive winds up “responsible” for making the choices, but there’s less to fail, too.
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