Files eventually stop copying to my SanDisk iXpand v2.0 USB 3.0 Flash Drive 64GB - Please help!

do you have icloud photo library turned on? If so the photos may not actually be stored on your phone and would have to be downloaded to copy to the iXpand. You can check settings or in the photos app if you see all photos instead of camera roll it means icloud photo library is turned on.