Michel,
I have discovered with the Freescale KL25z that copying the new firmware (I'm using the 20140530_k20dx128_kl25z_if_opensda.s19 linked from the KL25z platform page) on to the boot loader does not work on Mac, it works in windows, but I always "eject" my USB before unplugging it when in Bootloader.
I have updated the firmware on a number of KL25z boards successfully by following this process.
1. Hold down reset button and plug in USB to windows computer.
2. Drag and drop new firmware to BOOTLOADER drive
3. Right-click BOOTLOADER drive and select 'eject'
4. unplug when "safe to remove USB device" message appears
5. Plug KL25z board back in to pc (do not hold reset down this time.
6. New drive should appear as 'MBED'
Hope that helps.
Chris
When I plug the mini usb inth SDA, the green light flash 7 times, then stop, then flashing again 7 time....etc Nothing happen when I press the reset push button. Any idea?