Apple: Time machine will no longer back up. Message says "backup disc is not in Mac OS extended format. What do I do?

A simple 6 step guide for solving this common Apple problem quickly and effectively by GetHuman

You need to reformat and erase the disk:
From the Finder pull down on the Go menu to Utilities.
This will open the Applications > Utilities folder.
Plug in the backup drive.
Click Disk Utility
> Click on the second left-justified icon (not "Macintosh HD").
> Parition
> Partition Layout > 1 Partition
> Options ... > GUID Partition Table > OK
> Format: Mac OS Extended (journaled)
> Name: Backup 1
> Apply
> File > Quit
Click System Preferences > Time Machine
> ON
Unlock the lock if it is locked
> Add or Remove Backup Disk…
Choose Backup 1
In the upper right of the menu bar:
Pull down Time Machine to Back Up Now

From then on when your Time Machine disk is plugged in, Time Machine will make hourly backups.
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Author: BenF • February 17th, 2016

About this Step by Step Guide

After thousands of Apple customers came to GetHuman in search of an answer to this problem, along with others, we decide it was high time to publish instructions. Here are just a few examples of the types of questions we were being asked or problems reported by users like you: Apple: Time machine will no longer back up. Message says "backup disc is not in Mac OS extended format. What do I do?; I have purchased ***gbytes of cloud storage, it's still in storage but Im being told I have *byte...; My iPad mini was stolen tonight when someone broke in my house; AppleCare returned my iPhone * without my SIM.; My phone isn't connecting to servers like servers for apple services like email.; I am unable to download an app but apple tries to send a code but doesn't recognize my apple id.... So we put together this guide to try to help. It has 6 step(s) to it. It takes an average of 105 minutes to get through these steps according to other users, including time spent working through each step and contacting Apple. And we tried to include a checklist of information you'll need when you try to fix this yourself. Best of luck and please let us know if you successfully resolve your issue with this.

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