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It's my first time installing ubuntu. I have Windows 7 and 8 before in my laptop (Acer Aspire 4752z). After Ubuntu (12.04) installed in my laptop, the Windows boot loader didn't recognize my Ubuntu so I updated grub from a live USB.

And after grub updated, Ubuntu was booting perfectly but my Windows wasn't listed in the grub menu. So I follow someone instruction to update grub again. And after reboot, the grub menu didn't show anymore.

After searching about this problem on the net and trying to update grub again and again, I decided to delete the Ubuntu partition from live USB. And shortly after, I booted to Windows with Hiren boot and I used mbrfix. But i think it failed. Now i cant show my BIOS setup or boot from DVD or USB.

After that, the only screen i can see after booting is:

error: unknown filesystem
grub rescue>
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up vote 1 down vote accepted

The code that fits in the Drive MBR is very small. In the case of Windows, the bootloader is very simple and merely redirects to the bootsector of the partition with the boot flag set. Grub is a more complicated bootloader, but it's still limited by the space, so it has to load modules from a specific partition (the Ubuntu partition you deleted).

When Grub cannot find those modules (because the partition no longer exists), then you get a very limited grub rescue prompt. It can be used to load grub modules manually, but of course in your case, that's not possible because there is no partition. So there is really nothing you can do (that I am aware of) that will boot the remaining operating system (Windows).

In your case, the simplest way to fix it is to replace the Grub bootloader with a Windows bootloader. You mentioned you have problems booting from DVD and USB, in which case you'll have to figure that out or consider removing your drive to another machine. I think it's pretty unusual for a machine to no longer boot from USB or CD/DVD so I'd try troubleshooting that.

Then you can install the Windows bootloader from a Windows repair prompt (either by booting a Windows repair CD or a Windows Installation DVD) and running:

bootrec /fixmbr

Or you could boot an Ubuntu CD and install a bootloader with equivalent function...

sudo apt-get update
sudo apt-get install lilo
sudo lilo -M /dev/sda mbr

Lilo will give you a big warning after it's installed - but you can ignore that as when it is used as a Windows style bootloader, it works fine as written above.

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i cant booting from dvd rom so i cant use Windows Installation DVD.. and i cant boot from USB too.. is there any <rescue grub> command that i can use? I cant run into BIOS setup too.. i tried to press F2 before power button but its nothing.. – ropudito Mar 8 '13 at 3:33
Usually you press F2 or Esc after the BIOS posts (after the power button). What computer is it? (Brand model etc) – bcbc Mar 8 '13 at 4:08
Acer Aspire 4752z, and i cant go to bios menu after pressing F2 or Esc button.. and in the Acer startscreen the word "press F2 to enter BIOS menu"(or whatever it is) is gone.. usually it appear in the left bottom of screen.. – ropudito Mar 8 '13 at 5:41
Yes it's supposed to be F2 with this model. If that doesn't work, phone Acer or take it back to the store (if under guarantee) because it's basically bricked now. I just don't think a grub rescue prompt can boot windows. The only other option is to remove the hard drive and fix it from another computer. – bcbc Mar 8 '13 at 6:36
would you like to tell me how to do that? fix my laptop from other pc? – ropudito Mar 8 '13 at 15:11

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