Do you have Nautilus open when you mount the drive, and is Nautilus set to show preview thumbnails for video & images files? If so, can you switch off previews in Nautilus & see if the problem re-occurs? You say you have a lot of video & image files - maybe you have a video somewhere that has been converted to a .webp (animated picture) file instead of a .webm (video). If so, the thumbnailer would be reading every frame of the webp file, thus filling up your RAM. I don’t use Gnome, so I am not sure of Nautilus’ features, but this thread from 3 months ago may be helpful:
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