The built in boot loader is designed to work with windows, Linux and Mac OSx write to mass storage devices slightly differently which can cause corruption.
The fix for Mac OSx seems to be overwriting the firmware.bin with your own. So when using mbed you will need to rename the download .bin to firmware.bin and copy it accross, dont delete the firmware.bin on the DipCortex first.
OSx also likes to write system files to any directory or drive it touches which is unwanted in this case, this guide shows you how to disable this behavior http://apple.stackexchange.com/question ... ds-and-usb
For Linux If the method above doesn't work you can try using the dd command to copy it :
sudo dd if=your_new.bin of=/dev/disk/by-label/CRPx20DISABLD seek=4
More details can be found here :
AN11305: USB In-System Programming with the LPC11U3X/LPC11U2X - http://www.nxp.com/documents/software/AN11305.zip
AN11305v.1.pdf