Using Linux's fdisk to erase or fix a MBR?

Comatose51 · 03-10-2005, 11:50 PM

My friend somehow managed to write a MBR to his new IDE hard drive. Now his OS won't boot whenever it's plugged in because his compute reads the IDE drive first while his OS is on a SATA drive. He's tried changing order but doesn't seem to work.

I suggested using Knoppix to boot and use fdisk to erase the MBR. However, is fdisk capable of this?

Thanks.

tsphan · 03-11-2005, 12:06 AM

i do not know whethere fdisk can or not, but you friend could try and use the "dd" command to perfectly erase the entire mbr. I accidently did this on my computer and i lost my partition tables and everything on the MBR a month ago or so.

Comatose51 · 03-11-2005, 12:29 AM

How would you do that with dd?

I don't understand the command very well and have only really seen it used a few times before.

syg00 · 03-11-2005, 03:30 AM

Quote:

Originally posted by Comatose51
My friend somehow managed to write a MBR to his new IDE hard drive. Now his OS won't boot whenever it's plugged in because his compute reads the IDE drive first while his OS is on a SATA drive. He's tried changing order but doesn't seem to work.

This doesn't sound right.
Sounds like the BIOS isn't recognising the SATA as a primary disk if an IDE is present. The BIOS boots the primary disk - no ifs, no buts.
It doesn't cycle through to other disks.
Sounds like you mate needs a BIOS update - or a new motherboard.

Quote:

I suggested using Knoppix to boot and use fdisk to erase the MBR. However, is fdisk capable of this?

Won't help - not a function of fdisk, and see comment above.

camelrider · 03-11-2005, 12:34 PM

Or how about using the Knoppix CD to install LILO to the MBR of the IDE drive?

Comatose51 · 03-11-2005, 09:01 PM

Quote:

Originally posted by camelrider
Or how about using the Knoppix CD to install LILO to the MBR of the IDE drive?

Friend is afraid of Linux. Wanted to use the opportunity to show him how useful Linux is and then go from there... Alas, the lack of a quick answer persuaded him to install Windows XP on the hard drive and just migrate OS hard drives....

tsphan · 03-12-2005, 05:34 PM

you probably don't need the dd command anymore, but if the mbr does mess up for no reason,

dd if=/dev/zero of=/dev/HD count=1 bs=512

HD is your hard drive letter

though, i'm guessing you're not going to need that anymore, and it's best not to, it ruined my partition tables

syg00 · 03-12-2005, 06:03 PM

Quote:

Originally posted by tsphan
you probably don't need the dd command anymore, but if the mbr does mess up for no reason,
dd if=/dev/zero of=/dev/HD count=1 bs=512
HD is your hard drive letter

though, i'm guessing you're not going to need that anymore, and it's best not to, it ruined my partition tables

The reason it ruined your partition table is because you told it to.
The first sector is conventionally referred to as the MBR; as we all know - the MBR is *NOT* just the executable code we like to refer to as the boot-loader.
That executable code must be contained within the first 446 bytes. The description for (up to) 4 partitions follows - that is your "partition table".

All contained within that 512 byte sector - which the above command zeroes out as requested.

tsphan · 03-12-2005, 07:55 PM

Quote:

Originally posted by syg00
The reason it ruined your partition table is because you told it to.
The first sector is conventionally referred to as the MBR; as we all know - the MBR is *NOT* just the executable code we like to refer to as the boot-loader.
That executable code must be contained within the first 446 bytes. The description for (up to) 4 partitions follows - that is your "partition table".

All contained within that 512 byte sector - which the above command zeroes out as requested.

I found that out after I reformatted and some people told me that the MBR contained the partition tables

. Believe me, I learned a valuable lesson about the MBR that day.

syg00 · 03-12-2005, 08:05 PM

Quote:

Originally posted by tsphan
Believe me, I learned a valuable lesson about the MBR that day.

Would (probably) work with say BS=440.
Should be o.k., but I've never tried it - must set up that test machine and give it a go one day.

jiml8 · 03-12-2005, 08:18 PM

Quote:

you probably don't need the dd command anymore, but if the mbr does mess up for no reason,

NO NO NO!!!

This will wipe out partition table.

To use dd to clear the boot code from a standard DOS MBR, do it this way:

Code:

dd if=/dev/zero of=/dev/hda/ bs=446 count=1

This will wipe out the first 446 bytes, and stop just short of the beginning of the partition table.

Comatose51 · 03-12-2005, 11:34 PM

Damn.

New respect for *nix. dd is just too cool... too powerful to contemplate, yet so simple. That's what I love about *nix. I can do whatever hell I want with my computer, as long as it works in concept. Linux is free as in freedom AND beer!

tsphan · 03-13-2005, 08:46 PM

bs=446 eh?

Maybe i should try that out again! Just joking, I'm afraid to even touch that dd command now.

jiml8 · 03-13-2005, 09:41 PM

Quote:

bs=446 eh?

Yup. Partition table starts at the next byte.

DeeVoc · 07-29-2007, 09:45 AM

bump, this thread is probably ancient, but just helped me out.

Apparently grub does not like letting go of your mbr if/when you decide to go back to windows