It's scary that a read-only access damages files. I assume a lot of buffering is going on in the background, because audio is large, and has a lossless codec.
- format your USB drive as EXT4
if it doesn't work, create a swap file (I suggest to create it at the same USB drive, swap file size: twice RPi RAM size)
Thanks, I will try that. Just for the info It happen also with light mp3 to heavy DST and WAV audio file.