Only partial pictures transferring to flash drive from iphoto

For some reason, when trying to transfer photos from apple photos to my flash drive, some transfer as full photos and some look like they're loading from the top down but stop short and only transfer the top sections of the picture. The flash drive has plenty of space on it. When I copy and paste, they paste as "thumbs" and the files are way smaller, but the whole picture transfers. Anyone know why this would be happening? Thanks