Apple's New 21.5 Inch iMac Has Upgradeable Parts — That's a Huge Deal
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Apple iMac 2017 (mid) 21.5-inch SSD ,Ram & OSX Upgrades - ZD Laptop Service Centre
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Memory Upgrades for 2017 iMac 27-Inch
64GB RAM upgrade kit for 21.5-inch iMAC now offered by OWC - 9to5Mac
iMac Repair & Upgrade, Same Day Services
Amazon.com: OWC 32GB (2 x 16GB) PC19200 DDR4 2400MHz SO-DIMMs Memory Compatible with Mac Mini (Late 2018), 27" and 21.5" iMac (Mid 2017), and Compatible PCs (OWC2400DDR4S32P) : Everything Else
Memory upgrade: Apple iMac 2012/2013/2014 (Howto)
Teardown of New 4K 21.5-inch iMac Reveals Removable RAM and Modular CPU - MacRumors