What to do when Time Machine hangs on "Preparing for backup"

Although the Time Machine utility is one of the most advanced and demanded by all users who start on Mac, it is also true that it has its small 'big' failures when it comes to backing up the system, sometimes getting stuck or simply with some other corrupt copy that will not allow us to eliminate and that we will not be able to use later.

Issues aside, the truth is that 90% of the time it works great and completely transparent to the user, being the most reliable method to recover files or settings if we have to change Mac or our disk has damaged having to dump the copy of security.

In this case, we will talk about when it is perpetually in the "Preparing backup" state and what we must do to solve it in three steps.

1. Send the "in progress" file to the trash: To do this we will have to move to the partition or disk where we have the backup copy assigned and once inside the "Backups.backupdb" folder, look for said file and send it to the trash to later delete it. Of course, previously we will have to have stopped the backup in the cross that comes next to the progress bar within Time Machine, as shown in the image above.
   

2. Restart the Mac with the Time Machine Disk connected: As indicated, what we will do is restart the Mac with the Time Machine disk connected and this will cause Spotlight (the default file indexer in OS X) to check the disk again if necessary, which will take a while or if it has recently been done, it will not happen again.
3. Start Backup normally: At this point, we only have to start the backup again normally and see in the activity monitor if any disk or CPU use is happening when the dreaded "Preparing backup" appears. If we see that, on the contrary, it continues to remain 'stagnant' we will have to delve a little deeper into the subject.

To do this, the first thing to do will be to see if all the disk permissions and the disk itself are fine, so we will go to Applications> Disk Utilities and we will give it to Repair Permissions and Verify Disk later. The last option that comes to mind is that Spotlight has made an incorrect indexing of the main disk so the copy is not being carried out normally, as a last step we can try to enter > System Preferences> Spotlight> Privacy and add our main disk, in my case Macintosh HD and then remove it. This will force the system to revert to reindex the entire disk.