It is possible for Mac users to encounter a 'Time Machine backup failed' error while attempting to back up their computer with Time Machine after a major macOS upgrade. If the issue is indeed caused by a bug introduced with the update, users may have to wait for a newer version to be released that will fix the problem. Meanwhile, explore the solutions outlined below to see if any of them could help fix the 'Time Machine backup failed' issue.
Start by making sure that the file system of the external hard drive you are using is compatible with Time Machine. After all, most external hard drives are in either FAT or NTFS, the two file systems used by Windows computers. At the same time, even the native macOS file system APFS is incompatible with Time Machine. To check the format of the external hard drive, connect it, open Finder, select the drive from the left sidebar, and click on 'Ger Info.' Now, look at 'Format:' that is part of the list of general information. If it shows one of the unsupported file systems, then you will have to be reformatted as Mac OS Extended (Journaled) with a GUID Partition Table (GPT).
To start the process, you will need to open the 'Disk Utility' tool. Open Finder, go to Applications, followed by Utilities, and start Disk Utility. Select your external hard drive from the list on the left and click on 'Erase' from the top of the window. A pop-up window will appear. Here, you can rename the drive and change its format to the correct one. When you've made your selection, click on 'Erase.' Try to create a backup with Time Machine once more.
In the normal work process of Time Machine, several temporary extra support or service files are created. Due to an error or malfunction, however, these files may not be properly deleted and could start to pile up and cause unexpected issues. Users may have to remove such files manually.
The first step is to shut down Time Machine completely. Then go to 'System Preferences,' choose the Time Machine tab, and deselect the 'Back Up Automatically' option. Now to clean up the leftover files. Go to Finder, choose your external hard drive, and open the 'Backups.backupdb' folder. Locate the file ending in '.inProgress' and move it to the Trash. Empty the Trash bin and restart your Mac. When the machine boots, start Time Machine and try to create a new backup.