Error Code -1309????

I am trying to copy two mp4 video files (4.30 GB + 4.86 GB) to a thumb drive that already has a couple of mp4 files, but still has sufficient empty space (14.3 GB available).

At the end of the copy activity, I get this error message: "The operation can't be completed because an unexpected error occurred (error code -1309)."

Get Info for the two new and the two old video files on the thumb drive show they are to be opened with the Quicktime app and the error code message shows a yellow triangle at the base of the QT icon.

The two files already on the thumb drive have accompanying .srt files. The two I'm working with today do not. They were made as screen-capture videos using the QuickTime app. I am trying to put them onto the thumb drive for viewing on my TV.

Can anyone help me determine the problem? Thanks in advance for your help.