I have a late 2014 27-inch 5K Retina iMac, with a "Fusion Drive" composed of 1TB HDD and a 120GB SSD. This particular Mac has Yosemite (10.10.5) installed.

In order to do some testing of High Sierra, I wanted to create a new HFS+ Journaled partition using 200GB of the available 400GB, and once I install a bootable High Sierra, let High Sierra convert its installed partition to APFS.

Can I do that? Can the Fusion Drive setup have one Yosemite HFS+ Journaled boot partition, and one High Sierra APFS boot partition?

  • In a word… no. Now that it's mid-2019, though, it should be easy and affordable to buy a new SSD (I'd suggest the Micron/Crucial MX500, the 1TB size of which is selling for $120 on Crucial's website right now), format it in APFS, and go from there. If you like, you can leave the external 1TB HDD formatted as jHFS+ and continue using it as well. – jdmc Jun 7 at 4:17

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