time machine taking forever to prepare backup

Not sure how long "forever" might be, but Time Machine can take a number of hours in preparing after an operating system update to your Mac has been applied or a lot of new data has been added. When this occurs, Time Machine has to run a complete check of all the old backups before it can start a new backup.

Recently after an update to Catalina on my MacPro laptop, it took about 3-4 hours in "preparing" before the actual back up began. I've seen longer times on previous updates.

If you are using WiFi to back up your Mac to a Time Capsule, try to locate the Mac close to the Time Capsule so things will go a bit faster. Let "Preparing" run until the backup completes......or......until you see an error message.

If you stop the backup, then "Preparing" has to start all over again from the beginning, so avoid stopping the backup unless an error message appears.

If an error message appears, post back with the exact text of the message.

There could be a number of reasons, including:

• File corruption on the source disk.
• Faulty source drive.
• Corrupted backup set.
• Spotlight app running amok.
• etc.

Ref:

• If a Time Machine backup takes longer than you expect - Apple Support
• What to Do When Time Machine Is Stuck on "Preparing Backup" - Lifewire