can I use iMac 2013 as a secondary monitor
can I use iMac 2013 as a secondary monitor
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Use the 2013 iMac as a second display for what other Mac?
While it was possible to use the 2013 iMac running High Sierra or earlier as a display for an older Mac running Catalina or earlier.
Sadly it is not possible to use the 2013 iMac as a display for a newer Mac running a later macOS.
carefully read > Use your iMac as a display with target display mode - Apple Support