How can I transfer files from my drive to my SD card

I have an SD card that already have files on it so it doesn't need to be formatted. I have transferred the files to my drive but when I go to the file manager it doesn't show them all. I have been able to transfer a few but that is all from the phone to the SD card. So I know I'm doing it right. I have tried putting them on the drive first then transferring them to My drive but still doesn't show up on the phone in the file manager. So now I don't know what to do. I know I have more than enough space on the SD card so that isn't a problem. So now what do I try.