Grub Error 21 Reinstalling OS or Edit MBR

[nb123]

Hi,
I recently tried installing backtrack to my hard disk and I realized that I made a mistake specifying where to write the MBR to. I have windows xp on my primary HDD, and an external USB HDD that I just partitioned recently to install ubuntu and backtrack on. Now, I wrongly specified in the backtrack installer to write my MBR to /dev/sdb, which is my external HDD, although after searching online I realized I should have made it /dev/sda. Now, how do I uninstall backtrack to reinstall it again, or get the MBR on my primary. I tried deleting all the folders in the partition that I have backtrack in (sdb3), which I guess is a pretty newbie thing to do cos it didn't seem to delete it... How do I correct this problem? I've tried searching this forum and elsewhere, and they say that I could use the windows xp recovery fixmbr, but can I just reinstall backtrack in the partition sdb3 and write the MBR to the internal HDD?

What's happening now, is that when I load, I get the following message -

Grub loading stage 1.5
Grub loading please wait.....
error 21

However, when I hit Ctrl + Alt + Del, I can see my Windows XP and Ubuntu booting options. How do I just delete the data on the backtrack partition for a full reinstall? Will the backtrack installer give me this option? Do I have to remove the MBR from the sdb before writing to sda?

[devils casper]

Where did you install MBR earlier? You have installed Ubuntu and Backtrack in External Disk. Un-plug it and Boot up machine. Does Windows boot up?

[nb123]

MBR was installed on /dev/sdb which is my external hdd. When I disconnect it and start my computer, I still get the Grub error. I have to press ctrl alt del to get a boot options screen with choices for ubuntu and windows. After which, I select Windows XP and it boots up fine...

How do I go about removing the linux installation on the partition completely (cos I've already deleted many folders) and doing a full installation again, with writing MBR at the correct place?

Or do I have to do something else to the grub itself? Please guide me

(BTW devils_casper, I've been to Chandigarh and it's beautiful! )

[devils casper]

First of all, remove GRUB from the MBR of Internal Disk. Boot up from Windows Installation CD and select repair. It will drop you at command line prompt. Execute fixmbr. This will remove GRUB from internal disk and Windows will boot up on reboot.
Now, re-install Backtract in Existing Partition.

[nb123]

I'm a little intimidated to use fixmbr, I'm worried I'll lose data on my primary hard disk... Is there no other way? Also, will I have to reinstall Ubuntu along with Backtrack on the external hdd if I use fixmbr?

Also I forgot to mention that when I disconnect my USB external HDD, the ctrl alt del does not work anymore to get my boot options... does this mean that I have no choice but to use fixmbr?

[devils casper]

Fixmbr will remove GRUB only and there isn't any chance of data lost.
Where did you install GRUB of Ubuntu? Before installing Backtract, did you try to boot up Windows while keeping External HDD unplugged?

[nb123]

I'm not sure where the Grub of ubuntu was installed, but I followed the Ubuntu Installer, Manually created my own partitions with it, but let it do the installing.

Unfortunately I didn't try booting Windows with the external hdd unplugged before installing backtrack...

Ok, so should I just use fixmbr then?

[nb123]

Okay, I ran fixmbr and Windows now loads fine, with no grub error while booting. However, I reinstalled Backtrack and it's not recognizing it, I guess I reinstalled it on top of a previous installation, gonna have to reformat the partition? What software can I use to do this? Can I do this in Windows? Now my MBR is in the primary hard disk but if I reinstall Backtrack again, am I gonna have problems with the Grub again!!!?? Ahhhh

[devils casper]

You can use GParted to recreate partitions. Its available in Ubuntu LiveCD.
I think you have installed GRUB in MBR of Windows HD only because its default.
Does your Machine's BIOS support booting up from External Device?

[nb123]

yes it does, do you think I should put the MBR on the external HDD? This sounds like a good idea to me, cos it will leave my primary disk completely safe right?

Also, if I were to do this, I know how to do it with backtrack, but what about Ubuntu, I don't remember seeing instructions for it in the Ubuntu Installer. - (edited) I just read another post of yours and I'm currently downloading the alternate CD for Ubuntu installation, so I can choose to put Grub on external HDD.

Also, would I have to change the existing MBRs in the hard disk? Can I just delete the partitions where I currently have backtrack and ubuntu installed, and recreate the partitions putting the MBRs on the external USB? This will be great...