Files moved to WD 4T from MAC don't stay in WD

I’m clearing space on my crowded MAC computer hard drive by dragging MOV files from the MAC hard drive to a folder in my WD 4T external hard drive that I have open on my MAC desktop. The MOV video files appear to transfer to the WD normally and after they have loaded onto the WD a problem arises.

When I then drag the MOV files from the MAC hard drive to the trash on my MAC, the files that I just loaded onto the WD ext. hard drive vanish from the WD and appear in the trash along side the original files from the Mac hard drive.

What is causing this and how can I get the files I move to the WD external hard drive to stay there? When I move other files from the desktop on my MAC to the WD and they stayed there even after I trash the originals that were on the desktop.

For some reason it seems, files I drag from the MAC hard drive to the WD drive do not stay when the originals on the MAC are trashed. The files copied to the WD drive end up in the trash also as if they are still attached somehow or are just an alias or something.

@RockNRowBoat Do you have some type of Sync between the devices set up?

Not that I know of. I use iCloud to sync between my iPhone and my MAC but that shouldn’t effect moving files to an external hard drive, or would it?

I also have Dropbox Plus I use for storing and organizing files. It might have something to do with this as well.

I’m obviously not the most computer savvy person around which makes these kind of things even more frustrating.

Is there anyway I can check to see if anything is syncing the MAC and the WD external hard drive?

Thank you.