It's not clear just what you want to do. You say, "copy some pictures in Finder from the Pictures file fold to an external hard drive." Are the pictures that you want to copy being viewed in Photos or in Finder?
If the pictures are in Photos, then you should select them and then use File>Export>Export Originals (or export nn photos) and use the dialog to select the type, then the destination. Is that what you're doing? It's probably not best, by the way, to try to drag the pictures from Photos. You can select a picture and copy it and paste it into a folder, but you get weird file names that way, and no-telling what else.
Or are you looking at picture files in a folder in Finder on your Mac's internal drive? In that case you should be able to select the pictures and then just drag them to the appropriate folder on the external drive. If the files are corrupted, or in a format that is not acceptable, then following Yer_Man's suggestion will help.
So, what are you doing, exactly, that gives the error?