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How to zero a disk (especially MBR)?

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Top Forums UNIX for Dummies Questions & Answers How to zero a disk (especially MBR)?
# 1  
Old 03-15-2007
How to zero a disk (especially MBR)?

I'm confused. Originally I did

telinit 1
cp /dev/sda /dev/sdb

where sda is my boot disk and sdb is a USB disk. This probably copied my MBR.

Since /dev/sdb is 300GB and /dev/sda only 160GB I had a bunch of space left which I decided to experiment with by creating partitions of various sizes.

Then I did "telinit 1; cp/dev/sda /dev/sdb" again and I could not create additional partitions beyond the orginal 160GB like I had done before. Neither QTParted nor fdisk would let me partitition the 140GB of remaining free space like it had previously.

Now I run VMWare and I tell it I want a new virtual machine and I want it to use /dev/sdb. Well this does not work so well so I run "fdisk /dev/sdb" and delete all the partitions and then I use "dd if=/dev/zero of=/dev/sdb bs=1000 count=200000000" figuring this ought to be more than enough to zero out the MBR. Well dd comes back (very quickly) and says "dd: writing: /dev/sdb: no space left on device ... 65MB copied".

Why does it say that? I asked it to zero out more than 65MB?

Why is this not sufficient to delete GRUB from the entire disk?

I want to zero the entire 300GB disk. How do I do that?

Hmmm... so then I fire up VMWare and assign /dev/sdb to the new virtual machine which promptly gives me a grub prompt but nothing to boot. Arghhh! Why am I getting a grub prompt?

This is a problem because I don't know how to tell it (VMWare) to boot from the CD first instead of the hard disk. I want to install debian on /dev/sdb. Instead VMWare keeps running grub. I figure if I can zero out the entirety of /dev/sdb VMware will try to boot from /dev/sdb and fail and then boot form the CD.

Why do I want to do this? Because I've burned about 6 CDs of various versions of debian etch (net install) and they all register dump on me when I do a physical boot (boot without VMware) from the CD. I cannot boot debian sarge because it does not have support for my marvel network adapter. However, I have discovered that if I boot windows and run VMWare to create virtual disk for debian etch (on /dev/sda), I can boot and install debian etch. Unfortunately, I'm running out of disk space on /dev/sda and I need to use /dev/sdb!

Can someone please tell me now to zero a disk? (Or better yet, tell me how to change the device boot order for VMWare? -- but that is off topic!)

# 2  
Old 03-15-2007
i constantly reload the OS of my unix box, so i create a script like this


MBR=`df -hl | grep -v Filesystem | awk '{print $1'} | head -1 | sed -e 's/[0-9]//'`
echo GRUB is installed in $MBR
dd if=/dev/zero of=$MBR bs=512 count=1

make it executable & run it. It will wipe your master boot record.

#WARNING, think twice before you blow off the mbr.
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