USB-MSD bootloaders have become very popular in the last few years but OS manufacturers are not always making life easy with their enhancements to the behavior when external hardware driver are connected - Windows 8.1 update added a well known tripping stone which doesn't allow older mbed loaders to operate correctly, or be updated to newer versions with workarounds, for instance.
However Windows 8.1 is almost harmless in comparison to MAC OS X.
The uTasker serial loader (including KBOOT compatibility and USB-MSD in parallel) and available for all K and KL devices with USB was of course also not saved a certain amount of agony to ensure compatibility. For the low-down on just what the MAC OS X does with SD cards and USB-MSD loaders and workarounds implemented see the following development story:
Kinetis: µTasker Kinetis support
For the complete "out-of-the-box" Kinetis experience and faster time to market