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Would it be possible to put 3 SSD’s inside iMac? I was wondering because I have 3 x 256GB 2.5” SSD’s and I could of course put one inside of it and use 2 of them as external HDDs, but the speed won’t be great, and I don’t want them “hanging” outside… Is there a solution for this? === ===

Your system only offers two internal connections a SATA HD port and a PCIe SSD blade port. There is just no means to put them all three inside. The best you can do is take out your current HDD and using an adapter frame install one of your 256 GB SSD’s. But you’ll also need a in-line thermal sensor iMac Intel 21.5" and 27" (Late 2012-Late 2015) SSD Temperature Sensor