I've never had great luck with the mass storage flashing. It works fine, but boy is it slow. I bought a J-Link a few years ago and haven't looked back.

(It just takes a long time for the OS to recognize a USB device. And you have to press the reset button yourself in order to enter the bootloader. With something like J-Link, your build script can handle pressing the reset button and sending the code, saving you quite a bit of time between iterations.)