If Time Machine remains in the “preparing” stage on Mac
After the initial backup, Time Machine backs up only files and folders that have changed since your last backup. Time Machine displays a “Preparing...” message while it compares your current files and folders to those in the most recent backup.
The length of preparation time Time Machine requires depends on the amount of information on your Mac, the size of the backup, and the speed of the connection between your Mac and the backup disk.
The preparation stage can also take longer if:
The previous Time Machine backup was interrupted.
The Mac wasn’t shut down properly.
A disk was disconnected from your computer before being ejected.
A software update was recently installed.
You used your Mac for several days without performing a backup. This can happen if the backup disk was disconnected.
Antivirus software can also cause backups to require more time. Make sure your Time Machine backup folder, located on the backup disk and named “Backups.backupdb”, is excluded from virus scanning.