TAU is an integrated toolkit for performance instrumentation, measurement, and analysis. It provides a flexible, portable, and scalable set of technologies for performance evaluation on extreme-scale HPC systems. This paper describes alternatives for I/O instrumentation provided by TAU and the design and implementation of a new tool, tau_gen_wrapper, to wrap external libraries. It describes three instrumentation techniques – preprocessor based substitution, linker based instrumentation, and library preloading based replacement of routines. It demonstrates this wrapping technology in the context of intercepting the POSIX I/O library and its application to profiling I/O calls for the Global Cloud Resolution Model (GCRM) application on the Cray XE6 system. This scheme allows TAU to track I/O using linker level instrumentation for statically linked executables and attribute the I/O to specific code regions. It also addresses issues encountered in collecting the performance data from large core counts and representing this data to correctly identify sources of poor I/O performance.