Hidden data between MBR and first partition?
 

Hi all,

There's an option box to check in the Clonezilla backup disk to save what is described as the "hidden data between MBR and first partition?" At the risk of appearing ignorant (not something that's ever bothered me here) what exactly is the nature of this so-called "hidden data" and how much space does it take up?

tia,

cc.

In some sense the unit HDD are "built" from is cylinder.
On each disk (or using fdisk command) you can see how many cylinders disk has.

MBR is sector 0 in cylinder 0.
Any partition is start (and end) on the cylinder boundary.
So, even if your first partition started in the cylinder 1,
all but sector 0 if the cylinder 0 are lost. Or hidden.
if your first partition started from say cylinder 10, then all data in the cylinders 1-9 will be lost/hidden too

To be more specific, there are 62 sectors between the MBR and the first partition. Yes, that's a "track" worth, not a cylinder worth on modern disks, but sector 63 is considered the start of cylinder 1. And for a "classic DOS-style" partition table, the first partition needs to start here for historical reasons.
lilo has saved data into the sector(s) following the MBR for ages - grub also often uses them now (any boot virus worth the name would also use these sectors).

So is this first "track" known as "track 0"? In Acronis, there's a puzzling option which refers to saving the data in "track 0" so that refers to the first 62 sectors which obviously includes the MBR in sector 1?

Another thing: I believe there's the so-called 'magic number' (x0055 if I recall correctly) which signifies the end of the MBR. Is there a similar signature for the start of the first partition or is its start position simply 'taken for granted' by the O/S?

I have wondered myself if this area was used by some copy protection schemes. In Linux & XP, you can pretty much bank that the first partition will start on sector 63 (512 bytes * 53). You can use this to mount the first partition of an drive image backup:
losetup -fs imagefile.img -o $((512*53))
And this may come in handy if you accidently ruined your MBR partition table.

My toshiba laptop starts the first partition at 2046 for some reason. I don't know if this is Toshiba's doing or has to do with Vista.

Also keep in mind that there are other partition schemes. You might want to look up MBR in Wikipedia if you want to further your knowledge.

The area in the first 63 sectors can be used for other boot code, also. For example, I think GRUB puts stage 1.5 and 2 in the area after the MBR. In any case, you do not need to back up areas outside the partitions.

I don't think partitions start or end with any signatures or "magic numbers". The partition table tells the system where they are.

And what is the purpose of the MBR "signature" (0x55AA)? I have no clue.

The signature indicated a valid disk. Earlier versions of [cs]fdisk would refuse to open a disk that lacked these bytes. Maybe the DOS fdisk as well, I don't know.

These bytes exist in the first sector of (each) logical partition as well BTW.

Right!! Because an extended partition--and each logical partition--are part of a linked list in which each one has a partition table. In a sense, each logical partition is structured as a disk.

Indeed. And likewise, there's a /boot/grub directory in every Linux partition that uses Grub, each with its own menu.lst file! My main desktop runs 5 different distros and unfortunately, the Grub auto config routine of the latest install all too often incorrectly re-writes/augments the prior-existing menu.lst entries, giving rise to boot menu nightmares.

I've also had it so when I change the "default 0" to "default 7" to boot from sda8 instead of sda1, Grub ignores it and still boots the first partition regardless. Anyone else had this?

Maybe you should set up a "master" system, and only install grub on the later systems to the root partition - then chainload them all.
Or use something like gag which does the same thing.This has nothing to do with the partitions, merely the entries in the boot menu (count the "title" lines).

Well I'm posting my menu.lst file herewith. Let's face it: we can all use a laugh now and again. If anyone has a longer and/or more confusing one, I'd like to see it. Perhaps we could start a competition!