I disagree that I/O can be totally decoupled. The API that I/O functions generally adhere to is too weak for your application: on error, what do you want to happen? This pushes "I/O" functionality back into your logic, and that requires testing.
In this specific example, if you write the first file, but fail the second, you might want to undo the write to the first file. Or write another file to say you failed. Or write to a log. These sorts of things are important, and mocks can be a good way to fake it.
In this specific example, if you write the first file, but fail the second, you might want to undo the write to the first file. Or write another file to say you failed. Or write to a log. These sorts of things are important, and mocks can be a good way to fake it.