Dennis Faas over at Infopackets wrote a really good article and a step by step “how to” about sfc /scannow failures. You should probably bookmark this and print it out.
I remember this issue coming up here at WC. Well…here’s the link: https://www.infopackets.com/news/9620/how-fix-sfc-scannow-fails-wont-complete
Hope you never need it…but if you do, it’s really handy since he covers XP –> W8.