Cant write mp3 files to SD card

[Android Central Question]

I have a Galaxy S8 -- SM-G950F running Android 8.0.0
There was a Android software update on my phone a few weeks ago.
Since then I have been unable to write MP3 files to my SD card via the USB/charging cable to my laptop.
I dont get an error message, the Copy starts but stops about 3/4 of the way

Ive tried:
different USB port on computer
Double-checking that USB connection mode on phone is set to transfer files

I can remove the SD card from the phone and, using an adapter widget inserted into the computer I can write the mp3 to the SD Card
I have successfully written the same mp3 file to my MP3 player using the computer
So these two steps show that this isnt an issue with the SD Card or the computer.

My latest step was to upload the MP3 file to Google Drive from the PC, then open the Google Drive app on my phone and download it. The MP3 saves to internal storage but when I try to move it to the SD card the move fails.

I can play MP3 files from the SD Card via Samsung Music app

By the way there is 2GB of storage space left on the SD card.

Any ideas?

Barney.