Files degrade when moving to device

I have a 2 TB My Passport for Mac external drive.
I want to use it to store the tons of photos and video that my sons shoot.
Our Mac is full and needs some space.
When moving the files to the My Passport device they degrade. When I move a file that is 3.6 mb to the drive it turns into 1.7mb and looses resolution. Is there a way for me to just get an equal move out of this? I just want to save original integrity of the files.


How are you moving the files?

At first I was dragging and dropping. Now I’m trying to export.