How-To: Now's the right time to swap your old iMac's hard drive for a fast new SSD - 9to5Mac
Upgrading an older iMac's PCIe SSD: Third-party solutions that save you beaucoup bucks
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1TB SSD Replacement Kit for Apple iMac 27" A1419 (Late 2015) : Amazon.co.uk: Computers & Accessories
Can I upgrade iMac (Late 2015) fusion drive to SSD? | MacRumors Forums
Original 512GB SSD MZ-JPU512T/0A6 For Apple iMac 21.5-inch A1418 Retina(Late 2013 – Late 2015), iMac 27-inch A1419 Retina(Late 2013 – Late 2015), Mac mini (Late 2014), Mac Pro (Late 2013), MacBook Air