Acho que encontrei a resposta no fórum do Hacker de processos :
They're different concepts. If a process writes to a named pipe, that's I/O, but it has nothing to do with the disk(s) on your computer. Similarly if a process does I/O on a file, the contents don't have to be written out to disk immediately.
Portanto, a E / S inclui todas as transferências de dados, enquanto a leitura / gravação em disco pode ser imprecisa.