How do I tfr files from S8+ to usb flash drive connected via an OTG adapter?

[Android Central Question]

I want to tfr music I have on my Samsung phone to a USB flash drive without using a computer. I know how to tfr from USB flash drive to phone via OTG adaptor-thats how i got the music on my phone in the first place. But I cant seem to work out the reverse-transferring music on my phone to the flash drive via OTG connection..? I have selected the file stored in my Samsung Music but there's no option I can see to tfr it to a USB flash drive?
Di