You would be fixing that specific point in the security chain on that one hard drive, yes. But do note that something as low level as an mbr has super high access, and could even prevent/fake the mbr from being fixed again (if it is what was used to boot). Not saying this is likely, I am just not making any guarantees :-) . Make sure you boot from something else to fix it if you are worried about a corrupted MBR.
