Time Machine takes too long
When I’ve just restarted my Mac, Time Machine backups run normally. After a while, though, a backup will take a very long time to prepare, sometimes as much as a week. Why is that?
The most common cause of such protracted periods of preparing to back up is when Time Machine can’t find a usable database of changes which have been made to the files on a volume, known as FSEvents. It then has to check every file and folder to see if they have changed since the last backup, a process that can take days. This is called a ‘deep traversal’.
Another potential cause could be the backup folder itself. You can identify deep traversals and other problems with my Time Machine diagnostic tool, T2M2 (Free, eclecticlight.co).
Working out what is trashing an FSEvents database can be difficult. If you’re running any utility or other software supplied with a third–party external drive, try uninstalling that and restarting, as it can often clash with macOS with this effect.
If your backups are large, the chance of errors in them is increased. The way to address that and make them more spritely again is to archive your existing drive and start making a fresh series of backups from scratch to a new drive. That may seem drastic, but you’ll be surprised how quick backing up becomes after the initial complete backup of your Mac has been made. Some people do this every couple of years to avoid running into problems.