Hello, When an AntiVirus/Malware defender or utility such as MBRFilter https://blog.talosintelligence.com/2016/10/mbrfilter.html is active on the system , then trying to use syslinux to update the MBR would fail. PS D:\> D:\syslinux64.exe -a -m -f -i D: Writing MBR: Access is denied. Did not successfully update the MBR; continuing... PS D:\> echo $LASTEXITCODE 0 The issue here is in line https://repo.or.cz/syslinux.git/blob/HEAD:/win/syslinux.c#l509 eventhough we fail to update MBR but we still exit with 0. Need to have exit(1) to indicate that operation has failed. https://repo.or.cz/syslinux.git/blob/HEAD:/win/syslinux.c#l513 can have exit(1) aswell. thanks, Pradhap