To use a network backup over SMB, connect your Mac to an SMB share that is configured for Time Machine, then select that share in Time Machine settings. The setup depends on the server: a third-party NAS must explicitly support Time Machine over SMB, while a second Mac can advertise a shared folder as a Time Machine destination. These steps cover connecting and selecting the destination; they do not make an ordinary SMB folder Time Machine compatible. Apple’s backup disk guide

Check that the SMB destination supports Time Machine

First identify what hosts the backup. For a NAS, check its own setup documentation and enable its Time Machine feature for the share you intend to use. Apple supports third-party NAS devices that support Time Machine over SMB; a generic file share that merely opens in Finder may not appear as a usable backup disk. Apple also says that some non-Apple network disks do not support Time Machine. Apple’s backup disk guide

If another Mac hosts the destination, both Macs should use macOS 11 or later for this network backup setup. On the host Mac, open Apple menu > System Settings > General > Sharing, turn on File Sharing, and click its Info button. In Options, ensure Share files and folders using SMB is on. Add the folder or connected storage device to Shared Folders, Control-click it, choose Advanced Options, and enable Share as a Time Machine backup destination. You can optionally set a backup size limit there. Apple’s shared-folder instructions

Connect the Mac to the SMB share

Make sure the Mac you want to back up is on a network that can reach the NAS or host Mac. If the destination does not advertise itself to Time Machine, mount it manually in Finder: choose Go > Connect to Server, enter the SMB server address supplied by its administrator or shown in the device’s documentation, and connect with an account that has access to the designated share. Then select the intended share if Finder asks which volume to mount. Apple specifically notes that when a server or NAS does not provide Time Machine Bonjour notifications, you can connect to it over SMB and select the mounted network disk. Apple’s network disk guidance

Do not guess a network path or use a made-up Terminal command. The correct server name, share name, account and permission settings depend on the NAS or Mac hosting the folder. If you do not have them, obtain them from the device administrator or its vendor documentation. Keep the SMB share mounted while completing setup; Apple states that the Mac must be connected to the SMB server when setting up a network backup disk. Apple’s network disk guidance

Select the mounted destination in Time Machine

On the Mac being backed up, open Apple menu > System Settings > General > Time Machine. Click Add Backup Disk or the Add (+) button. Choose the advertised or mounted network destination, then follow the prompts to set it up. If macOS offers backup encryption, choose whether to enable it and retain the password somewhere secure; Apple says that password is needed to restore files from an encrypted backup. Apple’s Time Machine setup guide

If the network disk is absent, verify that the SMB share is mounted and that the NAS or host Mac has enabled Time Machine support for the specific share. If it still does not appear, the share may not support Time Machine; ask the manufacturer or administrator to confirm compatibility. Apple does not document a universal setting or command that can turn any SMB folder into a supported Time Machine destination. Apple’s backup disk guide

After selecting the disk, use the Time Machine menu to choose Back Up Now or monitor the initial backup. The first backup can take longer than later ones, and the network destination must be available for backups to run. Once configured, Apple says Time Machine automatically reconnects to a selected network disk when it is time to back up or restore. Apple’s Time Machine guide, Apple’s network disk guidance

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