Hi there !
Was just wondering if some kind soul could clear this up for me.
I play around with a lot of linux distros as I am always inheriting lots of old machines. My bootloader of choice is always grub, however some installations use lilo and the method I often use to remove lilo is to zero the MBR. Now I have read on LQ that if I use the following command the boot code will be zero'd but the partition table remains intact :

Whereas
will zero everything including the partition table record.

So if the latter is used, am I correct in assuming that not only will the boot code be wiped but also any record of how the disk is partitioned and formatted ( Partition Table record ?). Hence, if I accessed this drive using a live CD I would not get a readout of all the partitons and their respective formats when performing fdisk -l ? If my assumption is wrong what would be the output if I did run fdisk -l on a drive with the 512 bytes zeroed ?

Thanks,
Uncle-C

You've got it right. The second example erases the partition table and fdisk will report that the disk has no partitions.

Why erase the MBR when installing GRUB or LILO? Whatever you put in the MBR will simply overwrite what was there.

Thanks a lot for the reply Pixellany. The above is exactly what I thought
but when trying to install GRUB to re-write over LILO I was getting L99 99 99 99 99 errors on boot up. When I done a hexdump to view my MBR it showed that fragments of LILO were still installed. The way I eventually got grub to install properly was to zero 446 bytes and then re-install it. I was installing SLAX on a spare partiton and it uses LILO as the bootloader and gives you no alternative but to load it into the MBR of hda and not in any specified partition.

Thanks again for the help !

uncle-c

With all 512 bytes to 0, it must show the entire disk as unallocated space. Data aren't erased, so you could perhaps restore the deleted partitions using "testdisk".
But, if you put all the MBR to 0, then the disk label is deleted too.

Not understanding this.....I have always assumed that the MBR was defined as the first sector---ie the first 512 bytes.

On my system, I just made a label using "e2label". Using dd, I found this in the 3rd sector of the partition**---nowhere near the 1st sector / MBR.

**specifically, at an offset of OX478 from the start of the partition. This is in the 63-sector zone that precedes the start of the real partition data.

It is.
That’s because labeling a partition has nothing to do with the MBR. It is filesystem-specific (in this case ext2/3) and will undoubtedly affect whatever partition in which your filesystem resides.

As for masterclassic’s suggestion of using testdisk, you might instead backup and restore the partition table (and only the partition table).

As osor explained, masterclassic said "disk label".
Don't know what [s]he meant by that, but I can think of two candidates. The id bytes that Windoze stuffs in there (and the rest of the world ignores), or the "signature" bytes at the end (last 2 bytes) of the first sector.
Neither is a major issue any more - [c]fdisk used to abort if they couldn't find the sig bytes, but that's no longer the case.

Personally I always try to leave the part table and sig bytes intact.

It is this last that I mean, the last 2 bytes, that make the system to understand that the disk is formatted for an intel architecture, or for spark architecture, or for power-pc, & so on. This is the "msdos signature", usually needed for Linux/x86. This means that the partition table has this specific structure of 4 primary partition entries.

I think that most manufacturers now sale their disks with this signature already written. Obviously, if we install Linux or another system on an empty disk, it writes this signature in order to proceed to the disk partitioning and format.

Of course, I always prefer to backup the MBR before any partitioning work, rather than recovering Testdisk is a tool we can use if something goes wrong. It is contained in several live cds, as in Gparted livecd (from the terminal), and I think in the Ultimate Boot cd.