Can I use an iMac mid-2011 as an external monitor with the 2020 Mac mini?
Can I use an iMac mid-2011 (21.5") as an external monitor (it has a Thunderbolt port) with the 2020 Mac mini?
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While your 2011 iMac does actually support Target Display Mode.
Sadly all Mac's built after 2019 including the 2020 Mac Mini, do not support using an iMac as a display.
see > Use your iMac as a display with target display mode - Apple Support
"You can connect any of these older iMac models to another Mac from 2019 or earlier for use as an external display"