I'm using a netbook that had Windows XP, EasyPeasy 1.6, and Ubuntu 12.04. I was trying to replace Ubuntu with Arch Linux, but I ran into an error installing and I tried to reboot into EasyPeasy to look up what I should do. To my utter amazement and pleasure, I was greeted by the wonderful, friendly, welcoming GRUB repair prompt. error: no such partition. So I tried the following things, no luck. I feel like there's something simple I should be doing that I don't know about. My screen looked exactly like this (I took a picture with my phone, so I'd be able to post the output):
What should I do? Any help would be appreciated. Right now, I'm running from a Slax live USB drive as that's the only thing I can do with this computer right now.

Seems to me that grub does not have the modules loaded for the filesystems in use. Dont know if you can load modules from grub rescue but as you have a live system going just reinstall grub.

Either from the live system itself or from a chroot of easypease. Assuming ubuntu/arch is not ready for use yet. You're familiary with setting up a chroot enviroment?

So how do I reinstall GRUB from Slax? Slax doesn't use GRUB, so I can't just do grub-install /dev/sda. Is there something I can download to install GRUB to the MBR? How would I do it from a chroot in case I can't install from Slax?

I guess there is a package for grub for slax. At least a quick search showed some ways.

The chroot approach:
Boot live system (slax?)
mount the partitions of a system that contains grub to /mnt
do the following:
chroot /mnt /bin/bash
grub-install


With chroot its like working from within the system mounted on /mnt just like if oyu had booted into it. The proc,sys and dev filesystems are needed. Especially /dev.

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Your post would indicate that you were booting all three systems with Grub2 from Ubuntu. Before beginning the install, you should have booted EasyPeasy (or xp) and made sure the bootloader from either was installed to the mbr before trying to install Arch in case it failed, which it did. This is also why it is always a good idea to keep the medium you used to install a system, especially the primary which has the bootloader since most of the needed Grub files are on the partition.

If you aren't able to get it working from Slax, there are a number of ways suggested to install Grub2 at the link below. You would obviously need something with Grub2 on it.

https://help.ubuntu.com/community/Grub2/Installing

I have an odd problem. I tried doing mount /dev/sda6 /mnt, but I got this error message:
mount: special device /dev/sda6 does not exist
I know for a fact that my EasyPeasy installation is on /dev/sda6
I looked into /dev and sda6 in fact does not exist. There are no sda# devices at all.
However, strangely, when I boot into the graphical environment of Slax, I can just click on it on Dolphin's panel, which mounts it with no problems to /media/7b0dbeb5-0025-46c8-a657-5e7f4a945c16. So, apparently Slax calls it by a different name. What should I do?

Man! Whoever chose KDE as the default DE for Slax should be slapped! That thing is a slow, buggy mess. It takes 5 whole minutes just to boot into the desktop, and then it's almost unusably slow.

EDIT: I found out, it's sdb instead. So I followed your commands but ran into a problem.
Also, GRUB is installed to the MBR. It's not on any partition.

YEAHH!!! I finally got it working! I did a stupid mistake. I typed in chproc /mnt instead of chproc /mnt/dev. Now I can finally boot into EasyPeasy again. And as an added bonus, I get a nice verbose boot instead of a blank splash screen. I deleted my incomplete Arch partition before I started this just to minimize some risk. So, now that I've got it working, what should I do to install Arch without messing up GRUB again?

If you are going to install it to the former Ubuntu partition, you should be able to select to install Grub to the partition on which you put Arch. I've never used Arch so can't be sure but every distribution I've installed gave me that option. It's easy to overlook so make sure you read everything during the install.

Part of Grub (512 bytes) is in the mbr. The rest of Grub is on the partition. If you only had Grub in the mbr and nothing on any partition, it would not boot. On an Ubuntu system I have installed, the Grub directory is 4MB.

Very helpful and effective information. Thanks a lot.

I was able to sucessfully installed Arch Linux, and I'm very pleased with it. (Using it right now to post) My computer boots in 15 seconds! Arch replaced whatever version of GRUB I had with GRUB 2.02~beta2. I don't know why they're using beta, but it works fine and the OS prober worked correctly. On the GRUB screen, there are three entries for Arch Linux:
What's the difference between Arch Linux pkg-linux kernel and just Arch Linux? They both seem to do the same when I boot. Are they just two different kernel versions?