Got DOCX_LAST, XLSX_LAST and PPTX_LAST File Extensions on First My Passport Backup

My first backup of MS Office documents captured the correct folders but the documents themselves were unreadable since the file extensions were something the Windows 10 computer (or myself) did not recognize. The WD drive updated the backup every hour with the same results for about 5 hours. It was backing up run of the mill MS Office files; 75 GB on a 1 TB drive. Each time, the WD drive said the backup was complete but when I opened the drive, the files were unreadable. The actual files on my computer were always fine.

When I turned the machine back on this afternoon, the WD drive backup had the correct file extensions. Somehow magically it fixed itself.

Problem is, without knowing why this happened, I am afraid it might happen again and I can ill afford to lose these files to an unreliable backup drive.

Any help would be appreciated.