It’s a sad thing that after so many years of offering screen sharing as a native feature in OS X and then macOS, the remote-access service remains unreliable. I have two Macs in my house on the same network—one a laptop, one a desktop—and it’s often the case that the two can see each other and mount each other’s drives, yet can’t initiate screen sharing.
With Big Sur, I recently noticed that when I was unable to connect from one machine, the one that wouldn’t share stated it was being controlled by my other Mac already. (Big Sur offers more visible and obvious signs of remote screen sharing, as one of the subtle security improvements Apple added.)
If you haven’t used macOS’s screen sharing, it’s quite simple:
In the Sharing system preference, check the box next to Screen Sharing.
From another computer on the same network, find that computer in the Devices list in the Finder sidebar, select it, then click Share Screen in the upper-right corner. Log in with an account on the remote machine when it’s account screen appears.
To access outside your local network, Apple no longer offers its former direct method (Back to My Mac). You need to have a publicly assigned IP address on your computer or punch through a gateway, a much more complicated setup.
I attempted to disconnect that session from the sharing machine by selecting the Screen Sharing icon (two overlapping rectangles) in the menu bar and choosing DisconnectRemote Address. No go.
The solution is paradoxical: turn off screen sharing. This resets the service:
Open the Sharing system preference.
Deselect the box next to Screen Sharing.
When prompted, click Turn Off to confirm sharing.
Check the box against next to Screen Sharing to re-enable it.
You should find you can now connect from other Macs on the network.
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