How to Wirelessly Back Up Your Mac to a Windows Computer Using Time Machine
If you'd like to back up your Mac to an external storage device plugged into another computer on your local network while still being able to use the external storage device to store other files, follow these steps to do so.
Creating a Sparse Bundle Disk Image File in Disk Utility
Connect the external storage device to your Mac.
Open Disk Utility. You can do this by pressing ⌘ + Space and searching for it.
Click "File" in the Menu Bar and navigate to "New Image" > "Blank Image", or just press ⌘ + N on your keyboard.
Fill out the fields as specified below.
Save As and Name
Any name that is preferably all one word.
The external storage device.
Choose how much storage on the external storage device you would like to allocate to Time Machine. For instance, if you have a 64 GB external storage device, you may want to keep 12 GB separate from Time Machine, so you would type "50 GB" in this field.
macOS Extended (Journaled).
128-bit AES encryption (recommended). Then you will need to create a password. Do not forget this password.
Sparse bundle disk image. This is not to be confused with a "Sparse disk image".
Make sure that the "Size" field hasn't been reset, and then click "Save". Wait for the process to complete.
Allowing the Connection From Your Windows PC to Your Mac
Plug the external storage device into the Windows computer you wish to connect to over the network.
On the Windows computer, open File Explorer (Windows Key + E).
Find the external storage device and note down the drive letter. For example, if the name was "USB Drive (D:)", the drive letter would be "D".
Right-click the external storage device and click "Properties".
In the "Sharing" tab click "Advanced Sharing..." and enter your administrator password if prompted.
Ensure "Share this folder" is ticked.
In the next dialog box, select all of the users listed, apart from yourself, and click "Remove".
If your own user account wasn't listed, you need to add it to the list by clicking "Add..." and then type your username into the text field towards the bottom of the window, and press enter. It should automatically fill it with your full name if you have spelt it correctly.
Click "OK" to close the window.
You will be taken back to the previous window, in which you need to tick all of the boxes under the "Allow" column and then click "OK" twice and then "Close" to close all of the windows.
Open the start menu and type "cmd and press enter. A Command Prompt window will open.
Inside of Command Prompt, type "ipconfig" and press enter. A list of network information will open.
Look to the right of "IPv4 Address" and you will see an IP address. Note down this IP address.
Connecting Your Mac to the External Storage Device Wirelessly
On your Mac, open Finder and click the "Go" menu at the top and choose "Connect to Server", or press ⌘ + K.
Type "smb://" followed by the IPv4 address you noted down in step 14 in the previous section of this article, followed by the drive letter you noted down in step 3 in the previous section of this article. For example, if the IP address was "192.168.0.123" and the drive letter was "D", you would type "smb://192.168.0.123/d".
Once your Mac is connected, you will see the external storage device listed under "Locations" in the Finder sidebar, even though it is not physically connected to the Mac. It is now being accessed over your local network. You can place whatever files you want on the external storage device just by dragging it into it.
Click the external storage device in the Finder sidebar to open it and you will see the Sparse Bundle Disk Image file you created earlier.
Double-click it and you will be asked for the password you created in Disk Utility near the beginning of this tutorial. Once you have entered the password, tick the "Remember password in my keychain" box if you do not want to be asked for the password again on this specific Mac, and then click "OK".
You will now see the Sparse Bundle Disk Image file listed as a 2nd location in the Finder sidebar. Do not place any files in here — let Time Machine do the work. It is fine to store files outside of this Sparse Bundle Disk Image file on the external storage device though.
Setting Your Mac to Back Up to the Correct Location
Open Terminal. You can do this by pressing ⌘ + Space and searching for it.
Type the following into the Terminal window, including the quotes, and then press enter:
sudo tmutil setdestination "/Volumes/TheNameOfYourTimeMachineBackupDrive"
Replace "TheNameOfYourTimeMachineBackupDrive" with the name of the Sparse Bundle Disk Image file you mounted earlier.
Open System Preferences and click "Time Machine". You should see that the Time Machine destination has now been changed to the desired location.
If the Time Machine destination has not been changed to the desired location:
Open System Preferences.
Click "Time Machine".
Click "Select Disk...".
Choose the Sparse Bundle Disk Image file from the list of disks.
Click "Use Disk".
Initiating Your First Backup
Open System Preferences.
Click "Time Machine".
Click "Show Time Machine in menu bar" at the very bottom of the window.
Click the Time Machine icon in the menu bar at the very top of your screen.
Click "Back Up Now".