>Hello Viv;
>
>The CM.TXT file is the destinetion file.
>
>This error happen when all the file have been added.
>
>Thanks a lot for your time, I really apreciate it....
>
>Best regards;

Looks like the problem is in the use of:

For Each txtfile As FileInfo In topDir.GetFiles("*.txt", SearchOption.AllDirectories)

which would result in the source and destination paths being the same. Your actual directory/file structure is not clear but changing to

For Each txtfile As FileInfo In subDir.GetFiles("*.txt", SearchOption.AllDirectories)

should solve the problem ?

And, as suggested elsewhere you could use:

File.AppendAllText(Path.Combine(topDir.FullName, curFileName), File.ReadAllText(txtfile.FullName))

rather than the StreamReader....