Same problem was happening to me as mentioned by OP.
Howsoever after doing the following steps it worked like charm: -
Do these steps as mentioned in this video here. (Follow this video till 50 second mark) (Basically you have to delete the
.inProgress
File andreboot
your computer.)Now before connecting your external HDD for backup, go to terminal and execute the following code as mentioned in this answer. (i.e. Disable spotlight) (Execute only the first line of code)
Now connect your external HDD with your computer and start the backup.
Now please have patience, give it around 30 minutes
.
Even now if the backup is still stuck on "Preparing backup" then: -
Execute the second line of code given in the above linked answer (That is, enable the spotlight again) While "Preparing for backup" is still running .
Now in my case I waited for 45 minutes
after following the above steps and then finally my Mac started its backup.