Mac OS X 10.10 Yosemite rejects the format of the file system and puts it in a read only mode.
This issue was raised last October 24, 2014 at https://github.com/mbedmicro/CMSIS-DAP/issues/69 and is still open.
Work-arounds are available at https://developer.mbed.org/users/okano/notebook/mbed-on-yosemite/ but are not 100% reliable.
All mbed boards are affected, including Nucleo, Freedom, RedBearLab.
This is a tricky and potentially endless issue, as both the boards manufacturers and the OS developers can blame each other.
Any official answer from Nordic for the mbed-enabled boards?