When is the MBR created

#1
When is the MBR created or made? Does installing Windows XP or Windows 7 completely overwrite the MBR? I was wondering say in a situation where somebody had installed GRUB or GRUB-2 to the MBR, what will restore the MBR to its pristine condition? Will doing what the hard disk manufacturer calls a low-level format restore or delete the MBR.
 

mqudsi

Mostly Harmless
Staff member
#2
Yep. Whenever you install a new OS, it'll typically overwrite the MBR on the boot drive.

Installing GRUB won't restore the MBR, it'll overwrite it with its own copy.
 
#3
You say Windows will typically overwrite the MBR. I’m just trying to figure out what is the easiest and fastest method overwrite the MBR completely with absolute certainty. What about a wipe program that writes zeros and ones to the hard disk. What about FDISK/MBR. Will that overwrite the entire master boot record or only restore the parts of the master boot record related to DOS?
 

mqudsi

Mostly Harmless
Staff member
#4
The MBR is the very first 512 bytes of the hard disk. You can overwrite it with zeros very easily from any Linux or Mac with this command:

dd if=/dev/zero of=/dev/sda bs=512 count=1
And you can overwrite it from Windows by using the bootsect.exe with the /mbr flag