Cederic,
IAR's release runtime libraries do not support low I/O operations. Hence, the API printf(), putc() cannot be used in release mode. In your case, the __write function is referred by putchar which is called in the SysStd code
There are two solutions,
1) (Recommended) Do not use SysStd in release mode, instead use SysCallback and implement the callback functions to output to your I/O Console.
2) Implement the __write function. More information about the __write function can be found in compiler documentation in your IAR installation. It is under <IAR installation>/430/doc directory.
Hope this helps.
Vikram