Incase anyone ever needs the answer…I have a 27” 2010 iMac working in Target Display Mode with a M1 MacBook Pro using a miniDP to USB C adaptor that's capable of 4k60Hz.
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21.5" iMac Desktop Computer (Late 2013) + Battery Back Up + AppleCare + USB Card Reader + USB-C Adapter + 4-Port USB Hub - Walmart.com
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ME088LLA-A Apple iMac 27" Core i5-4570 Quad-Core 3.2GHz All-In-One Computer - 8GB 1TB GeForce GT 755M (Late 2013) - B
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Differences Between Late 2013 iMacs (Tapered Aluminum): EveryMac.com
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Making Sense of USB-C and Thunderbolt Cables and Ports on Your MacBook
Identify the ports on your Mac - Apple Support
SOLVED: USB-C ports are no longer working - iMac Intel 21.5" Retina 4K Display (2017) - iFixit
21.5-inch iMac (Late 2013) Review: Iris Pro Driving an Accurate Display
Apple iMac (27-inch, September 2013) review: New processors and faster Wi-Fi for Apple's slim desktop - CNET
Configure your own 27-inch Apple iMac (2013) at OWC
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Apple iMac (21.5-inch, 8GB RAM, 1TB Storage) - Previous Model : Electronics
iMac (27-inch, Late 2013) - Technical Specifications
Identify your iMac model – Apple Support (UK)
