I have a bootable Sierra SD card that doesn't show up in Startup Disk but it does mount in finder. I can boot other MacBooks with that SD card but on this one it will not work. I've also tied to burn InstallESD.dmg to a dual layer DVD and that doesn't show up in startup disks either but I can boot from an old Mountain Lion DVD just fine. I have also tried booting from a thumb drive no luck there either. I know it can handle Sierra as it's on there now but the user wants it securely erased and reinstalled.