I've a laptop with windows XP preinstalled. I've been able to repartition the 40 Gb HDD and now there's a 3 Gb partition for linux. During install of Ubuntu 5.1 I asked to mount it to /boot. I tried to place the rest of the os on partition of an external usb drive (160 Gb). I asked to mount that partition as /. Install went fine (I think ).

On boot up grub shows ubuntu and XP option. XP boots fine but when option Ubuntu is choosen, grub finds the kernel in /boot , does some booting up and then stops saying he was not able to find sda (where / is ) error 21 and then it is falling back to shell (?).
I've been reading a lot of posts on usb booting but in my case I cannot boot from usb via Bios(toshiba satellite 2450 S203). (that's why I had the tiny partition with /boot on hd0). I've read also some doc on grub(gnu grub and the legacy grub, in search for a solution for the error 21) and on the entries in fstab. No solutions found yet. In the thread 'Installing linux on external usb drive -> How about making a tutorial?' they speak of making an initrd.gz Is this a solution? I don't understand it fully.
Can anyone point me in the right direction.

1. What's going on in your system.
In order to boot successfully Linux kernel must be able to handle all the hardware involved in boot process. In your case, external USB drive. Generic kernel does not have this support built into kernel itself, this kind of hardware is handled via modules. These modules are located in /lib/modules on your root partition. You can see your problem now, I guess. To attach that external drive kernel needs to read that drive... Solution is to build a custom kernel with all neccesary features built-in or to use initrd.
2. However, you write you have there a 3 GB (Gb means gigabit) partition on hda. This partition is not tiny, for /boot it is huge. (My /boot is 15 MB, half empty.) I'd recommend to use it for /boot and / and leave the external drive for /usr or /home or maybe mount it at some custom mount point and use it for just data storage. 3 GB is enough to host entire Linux OS (in most cases).
Good luck.

Thanks for the insight you gave. I did not know the kernel is a basic one and the rest is in /lib.
The 3 GB :-) partition was just the maximum space I had to try it out. I could size it down,I suppose. I feared that my linux would grow in time due to installed programs ( as windows does with it's folder 'program files')
The whole endeavor was mainly set up to learn linux, so just for sports I'll try a little further.
So, from this point of view initrd is a good option ? Sounds like back to the man pages for me.
Thx for your time.

Plain root partition rarely grows over 100 MB, provided /usr, /home /tmp and /var are located elsewhere.