@echo off
echo fill HDD with zeros
rem --- generate 512 Bytes of 0 ---
echo n zerofile.bin>nul.txt
echo rcx>>nul.txt
echo 200>>nul.txt
echo w>>nul.txt
echo q>>nul.txt
debug < nul.txt > nul
del nul.txt

:loop
copy /b zerofile.bin + zerofile.bin zerofile.bin >nul
if not errorlevel 1 goto loop

del zerofile.bin
echo Done! free space was filled with zeros.
pause