Missing files from WD MyPassport after Mojave update

This weekend I upgraded my MacBook Pro to Mojave and everything was smooth till installation. After complete installation, when I connected WD drive I was greeted with message “Hard Drive doesn’t have enough storage”. Later I found some of my files are also missing. Didn’t find them in the trash or Backup drive. What should I do next to get my missing files back to the drive?

was the external drive connected to the mac when you performed the update? Typically I remove any external drive when performing OS updates.

What files are missing? If you do not have any backup of the files you would probably need to use some data recovery software to recover the files.