Question: I get this error:

```
Error: unknown filesystem.
grub rescue>

```

I'm not exactly sure what I did. I believe I installed Ubuntu over Windows on my Acer netbook. I then tried to install [Linux Mint](http://en.wikipedia.org/wiki/Linux_Mint), but it wouldn't start installing.

I turned the netbook off and back on. Now I get the error.

I have read a lot of other questions like this, but in my case I cannot boot a CD. If I put a Ubuntu CD or a Linux Mint CD into my external CD/DVD drive and change my BIOS to boot the CD-ROM first, it just gives me the same error screen.

**Update copied from comment dated 2012-05-26 02:54:29Z**

Here are some results from my commands: 

```
grub rescue>ls (hd0) (hd0,msdos5) (hd0,msdos1) 
grub rescue>ls (hd0,msdos5) unknown filesystem 
grub rescue>ls (hd0,msdos1) unknown filesystem 
grub rescue>ls (hd0) unknown filesystem 

```

Answer: 
This happened to me after I delete a partition that was located before the `/boot` partition.

To fix it, I ran an Ubuntu live USB stick, mounted the root partition to `/mnt` and the boot partition to `/mnt/boot`, and I ran this command (replace `/dev/sda` with the correct hard disk):

```
sudo grub-install --root-directory=/mnt --boot-directory=/mnt/boot /dev/sda

```
Your external CD/DVD is probably a USB device. You probably cannot tell BIOS to boot from it. When you start the ACER wacth for a message at the bottom (or top) of the screen saying something like "F2 to enter Setup, F12 for a Boot Menu". When you know what you need to press for the Boot Menu, put the CD in your external device, turn on or reboot the machine and slowly mash that key until you get a temporary menu allowing you to boot from the external CD.

You may need to use gparted from the CD to examine your hard drive and decide what to do to fix it so you can use it. That's beyond what I can describe here