how to find and/or re-install "Time Machine" on my Mac
Can't find and/or use my Time Machine. Need to BACKUP my Mac in order to re-install its OS
Earlier displays & monitors
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Back up your Mac with Time Machine - Apple Support
If Time Machine was working and it is not now then it may be your cable to the Time Machine External Hard Disk or the External Hard Disk itself. To see if the External Hard Disk can be seen, look in Finder to see if the Time Machine External Hard Disk shows in your sidebar. Let us know what you see in the Sidebar for next steps.
Also, if the Time Machine External Hard Disk is 5+ years old it may need to be replaced. ALL HD's fail, the question is when which cannot be estimated. If the HD is 5+ years old it would be wise to replace it. What I and many other experienced users recommend is the OWC Mercury Elite Pro