To understand what you are trying to do, you need to explain a lot more.
You want to repair your hard drive? You probably don't mean that literally. Very few people in the world are qualified to open a hard drive and mechanically repair it (it can be done, but typically only in R&D labs of disk manufacturers). You probably mean that your hard drive has caused some error messages. Are the problems hardware errors, or checksum errors detected by ZFS, or some functionality (like mount of fsck) not working?
Then you say that it displays errors. If you want us to help debug your problem, you need to be *WAY* more specific. Exactly what are the errors? Can you cut and past the exact error messages? There are zillions of possible problems with disks, storage stacks, and file systems, and just saying "errors" doesn't give us a hint of how to debug it.
To understand your problems, we also need way more information about your setup and the problems you are encountering. Is this a multi-disk system, or is this the only disk? Is it a real disk, or are you running a FreeBSD client in some virtualization environment? Is the disk an individual drive, or are you using RAID underneath? What is your file system (ZFS acts pretty different from other file systems)?
Why do you want to redo the MBR? If your MBR is faulty, then your errors are going to be that the disk partitions won't be found. If your MBR or your GPT partition table really have become damaged, and you know exactly how to reconstruct them (that's pretty unlikely, few people write down the exact partition layout), then an installation CD and booting in rescue mode is indeed one way to do it. But just rescuing the system is only part of the battle. You also need to find out what caused the damage in the first place, and make sure that it won't happen again.
