Reverse Syn -- Copy movies not watched.????

Not easily without having setup a system ahead of time. You may be able to write a macro that compares file titles on both drives, then copies those titles that do not match to a different Passport drive.

Try FreeFileSync. There are filter options that include time span.

https://www.freefilesync.org/

You could transfer the movies from the WD hard drive back to you computer.
You will be told the files are already on the computer and do you want to replace, skip or decide. REPLACE THE FILES ON your computer with the ones on the hard drive.Then highlight all the movies on your PC.
If they are all in one folder then highlight the folder and then choose "select All"
Go to the VIEW Tab
Choose "group by" drop down menu and select "Choose columns"
Put a check in the "Date accessed" box
Sort by date accessed. The ones not accessed will be together The ones you transfer will have the date of viewing or transfer on them.
Any way that is what I would try first.
Up to this point you have not done anything "bad" to your files
If it works as I figure than you can erase the movies on the Passport and copy the ones you want to the passport.

Can't be done as far as I know.
I also have a huge collection on a Seagate HD.
I have a folder called 'Viewed' on it. When I copy movies onto a thumb drive for watching on TV I then move those same files on the HD into the Viewed folder.