Write Audio To Disk From IO Unit

After a couple of days of tears & hair pulling I have a solution.

In my code and in other examples I have seen extaudiofilewriteasync was called in the callback for the remoteio unit like so.

** remoteiounit callback **

static OSStatus masterChannelMixerUnitCallback(void *inRefCon, 
                              AudioUnitRenderActionFlags *ioActionFlags, 
                              const AudioTimeStamp *inTimeStamp, 
                              UInt32 inBusNumber, 
                              UInt32 inNumberFrames, 
                              AudioBufferList *ioData)

{


    AudioUnitRender(engineDescribtion.equnit, ioActionFlags, inTimeStamp, 0, inNumberFrames, ioData);


    if(isrecording)
    {
        ExtAudioFileWriteAsync(engine->recordingfileref, inNumberFrames, ioData);


    }



    return 0;

}

In this callback I’m pulling audio data from another audio unit that applies eqs and mixes audio.

I removed the extaudiofilewriteasync call from the remoteio callback to this other callback that the remoteio pulls and the file writes successfully!!

*equnits callback function *

static OSStatus outputCallback(void *inRefCon, 
                               AudioUnitRenderActionFlags *ioActionFlags, 
                               const AudioTimeStamp *inTimeStamp, 
                               UInt32 inBusNumber, 
                               UInt32 inNumberFrames, 
                               AudioBufferList *ioData) {  


    AudioUnitRender(engineDescribtion.masterChannelMixerUnit, ioActionFlags, inTimeStamp, 0, inNumberFrames, ioData);

   //process audio here    

    Engine *engine= (Engine *) inRefCon;


    OSStatus s;

    if(engine->isrecording)
    {
        s=ExtAudioFileWriteAsync(engine->recordingfileref, inNumberFrames, ioData);


    }


    return noErr;

}

In the interest of fully understanding why my solution worked could somebody explain to me why writing data to file from the iodata bufferlist of the remoteio causes distorted audio but writing data one further step down the chain results in perfect audio?