My Mac osx10.8.5 desktop no longer backs up to Time Machine/Seagate 4TB external Disc
It makes the daily attempt to update, but gives an alert in a small window that the backup failed.
Earlier Mac models
Question marked as
Launch Disk Utility and run the First Aid routine on the external Time Machine drive. With luck the problem will be something repairable.
Please look for guidance here: Repair a disk using Disk Utility on Mac - Apple Support