On an iPad I previously simply dragged some videos on to the device in iTunes to copy them onto it. For some reason this did not work for my iPhone when I tried to do the same. You can of course add the media to the iTunes library and select to sync it on to the device that way. But if you don’t want to manage your media through iTunes that’s cumbersome.

To be able to do this without using the iTunes library, the option “Manually manage music and videos” needs to be enabled. You can find it on the Summary page if your device under the Options section at the bottom. be able to simply drag media files from the Finder to your device in iTunes. Click Apply after and now it should work.