So here's my situation:
I have two desktops. One is using Windows XP Media Center and my new desktop doesn't have an OS yet. I tried installing Ubuntu 11.04 on my new desktop but the dvd drive wasn't working. So I hooked up the harddrive via USB to my old desktop and installed Ubuntu on the new harddrive. I did that thinking I could then install the hdd in my new desktop and Ubuntu would work. When I removed the external harddrive and restarted my old desktop I got the following "error: no such device, grub rescue" I can connect the external harddrive and then grub will load and I can choose which OS to load. I have tried Windows recovery using fixboot and fixmbr; however, fixmbr does nothing and fixboot gives the following "FIXBOOT cannot find the system drive, or the drive specified is not valid". I have also tried copy e:\i386\ntldr c:\ but that gives me Access is denied. Can anyone please help me fix this. I am not wanting linux on my old desktop, and I would like it to load the OS automatically like before. Thanks

When you installed Ubuntu on the external drive, you probably chose to install grub on the MBR of the internal drive. Now, the grub configuration files show the first stage of grub on the MBR of the internal drive, which points to the rest of grub in the external drive /boot directory, in which the menu files are in the /grub subdirectory.

When you try to boot without the external drive, the first stage of grub in the MBR throws the error.

To repair the situation, plug in the external drive long enough to boot the OS on the internal drive. Then unplug the external drive.

Open a terminal and su to root, then run update-grub to re-write the MBR of the internal drive so that it points to the /boot directory on the internal drive. Then you should able to boot normally without the external drive.

Hi bigrigdriver, thanks for the reply. I am a newb at ubuntu and command prompt. You want me to boot my windows xp, unplug the external(I'm guessing as windows is loading?) and then.... you lost me. Could you explain how to open a terminal and su to root, update grub and re write MBR.

My apologies. I assumed you had more experience with GNU/Linux. I also apologize for misreading your post. So, I need to modify my suggestions, as well as be more detailed in the instructions so that you will understand what to do.

In re my first (erroneous) reading of your post, I make this revision to my response:
You probably installed grub to the MBR of the internal drive, which points to the rest of grub on the external drive. With the external drive unplugged, grub can't proceed because it can't find the rest of it's files on the external drive.
You should have installed grub to the MBR of the external drive.
For example:
Let's say the internal drive was shown as /dev/sda and the external drive was shown as /dev/sdb (it may have been shown as /dev/hd0 and /dev/hd1).
You should have chosen to have grub installed to the MBR of /dev/sdb so that when the external drive was re-installed in the older box, grub would then be in the proper location, with pointers to the rest of grub on that drive.

No. I want you to boot with the external drive plugged in, and boot into Ubuntu on the external drive.
In your gnome menu, look in the Utilities menu for terminal or console. Or, in your system tray, look for an icon that looks like a computer monitor (terminal). Click on the menu entry/icon to open a terminal. You will see the famous (infamous?) command line (with prompt). Su is the command to switch user (when the user name is given as a parameter to the command) and to switch to super user (root or sys admin) if no user name is given. You will then be prompted for the root password. Enter the password.
Once you are root, type the command grub-install /dev/sdb and press enter. That will install grub on the MBR of the external disk to point to the rest of grub on the external disk, then shut down.
Re-install the external disk in the old box and boot up. You should see a grub menu with both Ubuntu and Windows.

No apologies necessary. You are right that I installed the bootloader on my windows XP drive. I was able to do all that you mentioned, however; My external hdd that has ubuntu installed will be going into another desktop not the same one with my Windows XP. I am able to use the Ubuntu hdd in my new desktop now but my old desktop with windows XP still needs the Ubuntu hdd to boot from. Do you know how I can fix the boot loader for my Windows XP. I already tried fixmbr and fixboot but they don't work. Thanks again for the help.

I've never used xp but, from what I have read on various forums you will need an installation CD as Recovery CDs apparently don't have the necessary files. If you don't have the installation CD, post back. Someone will be able to tell you how to dd the mbr as windows appears to have trouble if another bootloader has data in the mbr.

If you have a win98 installation cd around, you can also use that to repair the xp MBR. The procedure is slightly different however. You should google up the command syntax to use with a win98 disk.

I am not sure but think I read that the microsoft site has a cd you can download to repair your xp mbr.