Flash drive plays in car but needs formatting for Windows computer

I want to add more music to my flash drive, which plays perfectly well in the car.  However, when I attempt to add music to it at my Windows 7 computer, I get an error that says it must be formatted.  If I do that, I’ll lose all the music I already have on it, I’m sure. 

What can I do?

If I do that, I’ll lose all the music I already have on it, I’m sure.”

Yup.

What can I do?

Reboot the pc and try again.

Try a different USB port.

Try a different pc. 

Once you find an option that works, BACKUP YOUR FILES!!