I have Windows XP installed on my internal hard drive (laptop), and I installed Ubuntu onto an external hard drive. Now, when I load up the laptop:
With the external drive plugged in, I can choose which operating system to load.
Without the external drive plugged in, I get a GRUB error 21.
I have found that GRUB needs to be configured onto the internal hard drive, but since it is Windows, I'm not sure how to configure it properly.
Any ideas?

Edit: I should have mentioned this first, but my laptop hard drive doesn't want to partition, and it doesn't really have the room for it anyways (60 gigs) for two operating systems, a backup, and games.

Yea it sounds like when you installed Ubuntu it installed Grub to the MBR of the external drive. Here is a good howto on working with Grub.

Question, when you installed Ubuntu did it ask you where you wanted to install Grub to, or did it just install it? The last time/s I installed Ubuntu it just installed Grub without asking, (which is one of the reasons I don't use it) but to be fair, I was installing it on its own drive without another OS on it, but even so it should ask.

Edit: O welcome to LQ

Unless you use the Advance tab Ubuntu install GRUB on hd0
Which drive was the first boot device when you install Ubuntu

Probably the easiest way would be a reinstall. At the point where you get asked where to install Ubuntu, switch to manual partitioning. You can keep Ubuntu on the external disk, but you should cut off a chunk of maybe 100 MB or so from the Windows partition. You should then format this small new partition with ext2 and set /boot as its mountpoint (these are options in the dialogue). That will have the effect that grub will install all its relevant files in that partition, and Ubuntu kernels will also end up there. That way grub should be able to boot Windows at least, when the external drive is not connected.

While you're at it, create two partitions on the external drive, set them to mountpoints / and /home respectively and format them with ext3. That way, all system files and programs will end up in one partition and all your personal files and settings in the other. This makes replacing the system at a later date without touching your personal files that much easier.

Robin

Personally, I would recommend you keep Ubuntu completely on the external. That way you can simply choose to boot it from the BIOS, or leave it unplugged and run just XP. That way you can take the external to another machine, and same applies.

From a Ubuntu terminal run "sudo fdisk -l" and post the results. That way we can refer you to the specific solution from saikee's link above, or tell you what to do.

It's awesome to see posts so soon.
Since my first post, I ran through some brick wall troubleshooting. First, I tried to set the BIOS to it's defaults, but that didn't change it, so I switched to an extra laptop hard drive, but that didn't do it. So, I decided that installing Ubuntu onto the spare laptop hard drive should set it to load the primary (Ubuntu, at least) hard drive.
Since then, the external hard drive ran into some bugs (I think it's a faulty) and I've tried reinstalling windows to cover the GRUB up.
Now that that's out of the way, the requested command (sudo fdisk -l) has been typed in, and the response back is:



Disk /dev/sda: 60.0 GB, 60011642880 bytes
255 heads, 63 sectors/track, 7296 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x14cb14cb

Device Boot Start End Blocks Id System
/dev/sda1 * 1 6992 56163208+ 83 Linux
/dev/sda2 6993 7296 2441880 5 Extended
/dev/sda5 6993 7296 2441848+ 82 Linux swap / Solaris

Disk /dev/sdb: 250.0 GB, 250059350016 bytes
255 heads, 63 sectors/track, 30401 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x000ce211

Device Boot Start End Blocks Id System
/dev/sdb1 1 30051 241384626 83 Linux
/dev/sdb2 30052 30401 2811375 5 Extended
/dev/sdb5 30052 30401 2811343+ 82 Linux swap / Solaris



Note: Since the external drive is bugging, and I'm using a different internal drive, this will not be the original, but I will post those when I am done with the first laptop hard drive, and run a live cd.

Edit Okay, so maybe that didn't go so well... In the effort of getting the GRUB to load properly, run windows internally, and run Ubuntu externally, I officially have no hard drive with windows on it. When I tried adding a small partition of Ubuntu on the internal drive to set the GRUB, I couldn't get any files that were still on the Windows partition! I could see them when I was running Ubuntu internally, but GRUB couldn't see Windows on startup, and now I can't run restore.

p.s. Thanks for the welcome, and the help!