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I have a laptop which is partitioned with Windows 7 and Fedora 18. I tried to delete F18 in Partition Manager in Windows 7 and then when I restarted my computer it would still go to GRUB launcher and show Fedora, Fedora with Advanced Options and Windows 7. Then I installed Ubuntu and when I restarted the system it said:

GRUB loading.. 
Welcome to GRUB!
error: disk 'lvm/fedora-boot' not found.
Entering rescue mode...
grub rescue>
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Executive Summary

This problem (very probably) has the same solution as this question; if you wish, you can immediately skip to the solutions there (take your pick).1

This should enable you to boot to both your Ubuntu and Windows systems successfuly.

1 However, since this is a very different form of the problem--the problem isn't the absence of GRUB in the MBR per se, but instead the presence of the old system's GRUB--I think this should be kept as a separate question, and not marked as a duplicate of that one.

What Happened, and How To Fix It

It appears that Ubuntu's installer detected the GRUB installation that belonged to Fedora (or parts of that installation) and decided to "play it safe" by not installing GRUB itself. Unfortunately, because the Fedora system doesn't exist anymore, its GRUB installation is non-functional.

The solution to this problem is to boot from an Ubuntu live CD/DVD/USB and install Ubuntu's GRUB boot loader. (This may or may not be a different version of GRUB than your Fedora system used. The issue here is that parts of the currently installed GRUB boot loader were located inside your Fedora system, and no longer exist. The corresponding parts of GRUB need to be installed in the Ubuntu system instead.)

To reinstall GRUB to your MBR and fix this problem, use any of the methods presented in either of these places:

In my opinion, the non-chrooting live terminal method or Boot Repair are usually easiest. Please note that you only have to use one method to achieve this. (Unless that fails. Then you might want to try another one.)

(Optional Reading) Some Technical Details About the Problem

The GRUB boot loader installs a small part of itself to a disk's boot record. Unlike some more minimalistic boot loaders (e.g., SYSLINUX, LILO), GRUB won't fit into a boot record, so the part it installs there serves to bootstrap into your main GRUB installation, which is located on a partition somewhere and typically is managed by one of your installed operating systems.

Your BIOS (or another boot loader) examines the boot record, executes the GRUB bootstrap code that's there, and that then hands off control to the fully-fledged GRUB installation in a partition.

Judging from your error message (mainly: error: disk 'lvm/fedora-boot' not found.), it appears that the GRUB bootstrap installed to your Master Boot Record points to the full GRUB installation in the Fedora system. That system no longer exists.

The solution, as explained above, is to make the GRUB bootstrap in the MBR point to the working GRUB installation in the Ubuntu system's partition (a partition which, unlike Fedora's LVM, does still exist on your system). This is the process colloquially referred to as "reinstalling GRUB to the MBR" or "repairing GRUB."

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None of these suggestions seem to address what can be done at the actual grub rescue> prompt itself, to continue booting into an alternate partition or USB drive where Linux temporarily is, without having to reboot and rely on outside tools. – Marcos Jan 21 '14 at 12:32
@Marcos I agree. An answer about that--or a new question and answer about troubleshooting and fixing problems from the GRUB rescue prompt itself--would be very helpful. I believe there are some common problems that cannot be resolved directly from the GRUB rescue prompt but others that can be. – Eliah Kagan Feb 26 '14 at 18:33

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