How do I put Ubuntu on a NON-flash external USB hard-drive?

Amazingly enough, circumstances have left me without a single USB flash drive, or a working CD-R drive. Also, because I moved about six months ago, I got rid of all my extra Ubuntu CDs that I used to get by mail. (cleanup win, hindsight fail) And yet, I need to get a live Ubuntu going to boot up a wonky desktop computer.

I tried using unetbootin to put a live CD install onto a portable USB hard-drive, but it won't boot from it (NTLDR missing error).

Is this because the disk is NTFS (which it is)? or for some other reason? Is there a difference between booting from a portable USB thumbdrive and a portable USB hard drive, other than potential performance?

There's no difference between a flash drive and a usb hard drive. Both can be used as a boot medium, and in the same way.

If you want to put the live system (installer) on the disk, the partition needs to be FAT32. NTFS cannot be read at this stage. So the partition you boot from (where the live cd contents are put) needs to be formatted as FAT32.

You can also install Ubuntu to the external hard drive, of course, just as you could with a flash drive. That's a different operation from using the drive as a live cd boot medium.

If you're talking about what I think you're talking about, running Ubuntu/Linux off of a external hard drive connected via USB then it's actually quite simple to do. Here are the steps, or rather, the steps I took.

Please Note: The following steps were tested using Ubuntu Version 9.10, but has not been tested with the later versions. Use at your own risk & discretion.

What You Will Need

1. A Computer with Internet access.
2. A LiveCD or LiveUSB with Ubuntu.
3. An external Hard Drive with USB capability.

What To Do

1. Open up your computer and remove the Hard Drive.
2. Plug in your external USB Hard Drive via the USB cable.
3. Stick in your LiveUSB or LiveCD and then boot up your PC.
4. Open up the boot menu, and choose to boot from the LiveCD/LiveUSB.
5. During the installation process you should your external hard drive listed, install Ubuntu to that.
6. Finish the installation process, turn off your PC, and put your other hard drive back into your computer.
7. Reboot your computer, go to the boot menu and select your external hard drive and attempt to boot from it. If it does congratulations, you now have an external hard drive with a full fledged Operating System on it.
8. Enjoy your external hard drive running Ubuntu/Linux! Please do let me know if this helps you! If not let me know about that too. :)

You can use $ sudo aptitude install ubiquity

Install that on your machine (that's the-ubuntu-installer), then go through installation instruction make sure you set up partions on the usb-drive & at the last step select advanced and make sure the bootloader is installed on to usb-drive as well.

This will give you portable-ish Ubuntu on usb-drive. (ext4 etc.)

Currently the only tool that works for installing Ubuntu on NTFS is wubi but that's to be installed along with Windows.

If the computer is attached to the Internet, then use Wubi.

http://wubi-installer.org/