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A disk read error occured
Press Ctrl+Alt+Del to restart

Often you come across the above message when you update operating system in dual boot system: Windows 7 and Linux like Ubuntu 13.04.
Your Ubuntu is working well but cannot boot Windows 7. You can correct this problem just by modifying the grub.cfg file in Ubuntu as follows:

Note that you cannot directly modify grub.cfg file. But there is a way to do it as explained below.

First you need to check the grub.cfg file. In Ubuntu13.04, you can see this file by clicking Files/Computer/boot/grub/grub.cfg

The grub.cfg file starts with the following text.

#
# DO NOT EDIT THIS FILE
#
# It is automatically generated by grub-mkconfig using templates
# from /etc/grub.d and settings from /etc/default/grub
#
 ...........
.............

Under "### BEGIN /etc/grub.d/30_os-prober ### " you may see the insmod as ldm.
The set root was set to /ldm/(lotofhexnumbers)/volume1.

In my system, Windows 7 is installed in first partition and Ubuntu in second partition drive. 

You can edit above two lines from the root.
Go to terminal sudo gedit /boot/grub/grub.cfg. Change the two lines as given below:

insmod part_msdos

set root='(hd0,msdos1)'

One way to modify grub.cfg is to edit 40_custom file under etc/grub.d/40_custom
But the easiest way is to use the command sudo -w /boot/grub/grub.cfg 

Save it, reboot the system and selecting the Windows will boot normally.