#include <File.au3>
$aDst_Files = _FileListToArray(@ScriptDir & '\Files', '*.txt', 1, True)
If @error Then Exit MsgBox(48, @ScriptName & ' - Error', 'Unable to get files list.' & @CRLF & @CRLF & 'OK ==> EXIT', 0, Default)
$sSrc_File = @ScriptDir & '\File.txt'
$sSrc_Read = FileRead($sSrc_File)
For $i = 1 To UBound($aDst_Files) - 1
$sDst_Read = FileRead($aDst_Files[$i])
$hFile = FileOpen($aDst_Files[$i], 2)
FileWrite($hFile, $sSrc_Read & $sDst_Read)
FileClose($hFile)
Next