Export Datagridview data to existing excel sheet

You are trying to use the same worksheet?  As i Know, one worksheet seems to contain 65 000 rows maxly. If you truly wish to continue that,  then you can merge worksheets. You can easily achieve it with  Spire.Xls. Here is an article from a blog: http://c-sharp-paradise.blogspot.com/2012/04/how-to-merge-excel-files.html, which helps a lot.  But please notice it is a commercial ware for developers. SO Choose when You need. I used it to develop my sulotion and feels fine. So I recommend. But Be Careful.  Best Luck.