just replace the files in HDM/C. you should also try deleting the ndure/C and E folders and redoing Ndure. this time choose only core files, unknown kernel and then create XBOXHDM installer. then transfer over the C and E it creates and try to softmod again.
im not sure why, but it seems that some people have more luck this way. i have used the method in the tutorial dozens of times and it always works for me, but some people have problems....
if installing ndure still fails(error 21), try restoring it to retail buy choosing option 2, then type XBOXHD and create xbox HDD from scratch.