dd of DOS MBR wanted (to boot dos on hdb)

dd of DOS MBR wanted (to boot dos on hdb)

Post by Homer Ja » Sat, 01 Jul 2000 04:00:00



Can someone email me a copy of their DOS MBR? Mine has evidently
gotten erased from hdb during my fiddling. fdisk /mbr like to do
its work on hda and I do not want to have to rejumper my disks just
to install an MBR.

As a further explanation, I want to do this so that I can boot DOS
on hdb1. It does not seem to work if I just point lilo (or the minix
monitor on hdb2) to hdb1. I noticed that to boot redhat on hda1, the
minix boot monitor requires I point it to hd0, which is equivalent
to just hda in lilo. (Lilo of course seems ok with hda1.) It got me
thinking that maybe I should point monitor (or lilo) to hd5 (hdb).

Right now, if I point lilo to hdb1 or minix monitor to hdb6 (=hdb1),
I get "invalid system disk". Inserting a dos boot floppy works ok
and I can switch to the C: filesystem just fine. I want to do this
front the bootloaders however.

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dd of DOS MBR wanted (to boot dos on hdb)

Post by Rod Smi » Sat, 01 Jul 2000 04:00:00


[Posted and mailed]



Quote:> Can someone email me a copy of their DOS MBR? Mine has evidently
> gotten erased from hdb during my fiddling. fdisk /mbr like to do
> its work on hda and I do not want to have to rejumper my disks just
> to install an MBR.

Overwriting the ENTIRE MBR with one from a random computer is a Bad
Idea, because the MBR includes partition table information. So unless
your "donor" has a drive that's EXACTLY the same size and has partitions
defined in EXACTLY the same way, you'll get incorrect partition
information. If your /dev/hdb partition information is missing anyhow,
then you're in deep trouble. If you know the exact sizes of that drive's
partitions, you may be able to reconstruct it.

Quote:> As a further explanation, I want to do this so that I can boot DOS
> on hdb1. It does not seem to work if I just point lilo (or the minix
> monitor on hdb2) to hdb1.

DOS cannot normally be booted from /dev/hdb. There are some boot loaders
(like the Ranish Boot Manager,
http://www.users.intercom.com/~ranish/part/) that claim to allow this,
but I've never tried it.

--

http://www.rodsbooks.com
Author of books on Linux networking & multi-OS configuration

 
 
 

dd of DOS MBR wanted (to boot dos on hdb)

Post by Svend Olaf Mikkels » Sat, 01 Jul 2000 04:00:00



>Right now, if I point lilo to hdb1 or minix monitor to hdb6 (=hdb1),
>I get "invalid system disk". Inserting a dos boot floppy works ok
>and I can switch to the C: filesystem just fine. I want to do this
>front the bootloaders however.

Is C: a FAT16 or a FAT32 partition?
--
Svend Olaf
 
 
 

dd of DOS MBR wanted (to boot dos on hdb)

Post by Homer Ja » Sat, 01 Jul 2000 04:00:00


Quote:> >Right now, if I point lilo to hdb1 or minix monitor to hdb6 (=hdb1),
> >I get "invalid system disk". Inserting a dos boot floppy works ok
> >and I can switch to the C: filesystem just fine. I want to do this
> >front the bootloaders however.

> Is C: a FAT16 or a FAT32 partition?

It's FAT16. I think now, however, that this is an ill-conceived idea
all together. I should have been more clear about why I though the
DOS MBR was fried, my only evidence being the inability to boot DOS
on hdb1 and knowledge that I had been fiddling with the MBR in the
past. However, I think now that the DOS MBR is intact, but either:
-DOS MBR is _incapable_ of booting an OS on hdb
-DOS MBR can boot hdb, but as written it points to hda
I did not specify that I wanted an MBR that pointed to hdb1, which
I should have done. Also, overwriting the entire boot sector, as
was pointed out, overwrites the partition map. One could overwrite
the portion of the MBR _before_ the map, however. But, I'm guessing
now that the DOS MBR simply can not boot hdb (without a third-party
utility). Anyone disagree?

Could I install a LILO MBR on hdb that'll point to hdb1 and so
boot DOS?

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dd of DOS MBR wanted (to boot dos on hdb)

Post by Svend Olaf Mikkels » Sat, 01 Jul 2000 04:00:00



>> >Right now, if I point lilo to hdb1 or minix monitor to hdb6 (=hdb1),
>> >I get "invalid system disk". Inserting a dos boot floppy works ok
>> >and I can switch to the C: filesystem just fine. I want to do this
>> >front the bootloaders however.

>> Is C: a FAT16 or a FAT32 partition?

>It's FAT16. I think now, however, that this is an ill-conceived idea
>all together. I should have been more clear about why I though the
>DOS MBR was fried, my only evidence being the inability to boot DOS
>on hdb1 and knowledge that I had been fiddling with the MBR in the
>past. However, I think now that the DOS MBR is intact, but either:
>-DOS MBR is _incapable_ of booting an OS on hdb
>-DOS MBR can boot hdb, but as written it points to hda
>I did not specify that I wanted an MBR that pointed to hdb1, which
>I should have done. Also, overwriting the entire boot sector, as
>was pointed out, overwrites the partition map. One could overwrite
>the portion of the MBR _before_ the map, however. But, I'm guessing
>now that the DOS MBR simply can not boot hdb (without a third-party
>utility). Anyone disagree?

>Could I install a LILO MBR on hdb that'll point to hdb1 and so
>boot DOS?

If we assume that BIOS is set to boot from hda:

Boot Windows 95/98 from disk no 2 or later with Lilo
Method 2.
----------------------------------------------------

Explained with an example. If a disk with DOS/Windows 95/98 is
installed as hdb, and no primary FAT partitions exists on
hda, the following method can be used:

First copy the boot sector to /boot:

dd if=/dev/hdb1 of=/boot/bootsect.b1 bs=512 count=1

Open Midnight Commander and find the file bootsect.b1.
Select View (F3)
Select Hex  (F4)
If FAT16: Go to offset hex 00024.
If FAT32: Go to offset hex 00040.
The value will be hex 80.
Select Edit (F2)
Change the byte from hex 80 to hex 81.
   (this is the embedded BIOS disk number)
Select Save (F6)

(My copy of MC will do Edhex when EdText is shown).

In stead of MC, any hex editor can be used.

Make sure the file size is still 512 bytes.

Then add to lilo.conf:

other=/boot/bootsect.b1
        label=win98

and run Lilo.

--
Svend Olaf

 
 
 

dd of DOS MBR wanted (to boot dos on hdb)

Post by Homer Ja » Sun, 09 Jul 2000 04:00:00





[...]
> >> >Right now, if I point lilo to hdb1 or minix monitor to hdb6 (=hdb1),
> >> >I get "invalid system disk". Inserting a dos boot floppy works ok
> >> >and I can switch to the C: filesystem just fine. I want to do this
> >> >front the bootloaders however.
[...]
> >past. However, I think now that the DOS MBR is intact, but either:
> >-DOS MBR is _incapable_ of booting an OS on hdb
> >-DOS MBR can boot hdb, but as written it points to hda
[...]
> If we assume that BIOS is set to boot from hda:
> Boot Windows 95/98 from disk no 2 or later with Lilo
> Method 2.
> Explained with an example. If a disk with DOS/Windows 95/98 is
> installed as hdb, and no primary FAT partitions exists on
> hda, the following method can be used:
> First copy the boot sector to /boot:
> dd if=/dev/hdb1 of=/boot/bootsect.b1 bs=512 count=1
> Open Midnight Commander and find the file bootsect.b1.
> Select View (F3)
> Select Hex  (F4)
> If FAT16: Go to offset hex 00024.
> If FAT32: Go to offset hex 00040.
> The value will be hex 80.
> Select Edit (F2)
> Change the byte from hex 80 to hex 81.
>    (this is the embedded BIOS disk number)
> Select Save (F6)
> (My copy of MC will do Edhex when EdText is shown).
> In stead of MC, any hex editor can be used.
> Make sure the file size is still 512 bytes.
> Then add to lilo.conf:
> other=/boot/bootsect.b1
>         label=win98
> and run Lilo.
> Svend Olaf

I finally got around to trying this and I thought it beared
mentioning that it worked *perfectly*. This is certainly the
way I needed to boot DOS on a second disk using lilo. Thanks Svend!
Homer Jay

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RedHat 6.1
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