In IOS is only Internal Storage ,but in Android is diffrent since some android device can use sd-card

theClass.prototype.WriteLog = function(theJSON){

    Ti.API.info('WriteLog = ' + new Date() );
    var thisClass = this;
    var jsonDir = '';
    var dateObj = new Date();
    var month = dateObj.getUTCMonth() + 1; //months from 1-12
    var day = dateObj.getUTCDate();
    var year = dateObj.getUTCFullYear();
    var theDate = year + '' + month + '' + day;


    if(thisClass.Parent.OSName == "iphone" || thisClass.Parent.OSName == "ipad"){
        jsonDir = Ti.Filesystem.getFile(Ti.Filesystem.applicationDataDirectory ,'json');
        //write into a file
        this.jsonFile = Ti.Filesystem.getFile(Ti.Filesystem.applicationDataDirectory,'json','pocsjson'+theDate+'.txt');

    }else{
        jsonDir = Ti.Filesystem.getFile(Ti.Filesystem.externalStorageDirectory,'json');
        this.jsonFile = Ti.Filesystem.getFile(Ti.Filesystem.externalStorageDirectory,'json','pocsjson'+theDate+'.txt');
    }

    //create folder
    if (! jsonDir.exists()) {
        jsonDir.createDirectory();
    }

    //serialize
    this.jsonFile.write( JSON.stringify(theJSON), true); // write to the file
     //this.jsonFile.deleteFile(); // delete the file

    //read that file       
    var contents = this.jsonFile.read();
    Ti.API.info('jsonFile.read() as a blob: '+contents); // useful if contents are binary

};

tiapp.xml Setup

<android xmlns:android="http://schemas.android.com/apk/res/android">  
    <manifest>
        <application android:theme="@style/Theme.Titanium"/>
            <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
            <uses-permission android:name="android.permission.STORAGE" />
            <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
        </manifest>
</android>