How to fix ubuntu showing grub rescue at booting

When I reinstalled ubuntu from scratch, the machine sometimes throws the following error:

error: no such partition.
Entering rescue mode...
grub rescue>

It seems that the system enters into grub rescue mode when grub is installed in a drive other than /dev/sda.

Solution

Step 1. list file systems

grub rescue> ls
(hd0) (hd1) (hd2) (hd1,msdos6) (hd1.msdos5) (hd2.msdos1)

Step 2. find the file system for grub to be installed

grub rescue> ls (hd1,msdos6)
(hd1,msdos6): Filesystem is unknown.
grub rescue> ls (hd1,msdos5)
(hd1,msdos5): Filesystem is unknown.
grub rescue> ls (hd2,msdos1)
(hd2,msdos1): Filesystem is ext4.

Step 3. configuration

grub rescue> set root=(hd2,msdos1)
grub rescue> set prefix=(hd2,msdos1)/boot/grub
grub rescue> insmod normal
grub rescue> normal

Step 4. permanently fix the configuration

$ sudo update-grub
$ sudo grub-install /dev/sda

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Reference

1. How to boot from grub rescue?