How do I reinstall macOSX on a legacy 2015 iMac with two internal drives?
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Apparently, your Mac was originally outfitted with a Fusion drive, a combination HDD and SSD fused in software to appear as a single, faster drive. That fusion has been split. Before you reinstall macOS, you'll want to fix the split Fusion drive. Apple provides guidance for that here: How to fix a split Fusion Drive - Apple Support
Once the Fusion drive has been repaired you can reinstall macOS by following the guidance found here: How to reinstall macOS - Apple Support