Back up to a shared folder with Time Machine on Mac

You can share a folder on a Mac that other Mac computers on your network can use as a destination for Time Machine backups.

On the Mac you want to use as a Time Machine backup destination, choose Apple menu > System Preferences, then click Sharing .
Select the File Sharing tick box.
Click Options, make sure “Share files and folders using SMB” is selected, then click Done.
Click the Add button at the bottom of the Shared Folders list.
Navigate to and select the folder you want to use for sharing, then click Add.
Control-click the name of the folder, then choose Advanced Options.
If available, click the “Share over” pop-up menu, then choose SMB.
Select “Share as a Time Machine backup destination.”
If desired, select “Limit backups to,” then enter a size.
Click OK.

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macOS User Guide