I am trying to load Ubuntu 10.04 onto a Seagate 500 GB External Hard drive. I recently used a small flash drive to install Ubuntu 11.10 onto my computer, but I seem to be having trouble doing some things that I need to do, such as sync my iPod. I have found that my only option to downgrade would be to do a fresh install, so I want to make my USB drive into an install disk. I have already downloaded Unetbootin, and the ISO file for 10.04, but my hard drive doesn't show up. I read reports that if this occurs to format it to FAT32, which I did using the installed Disk Utility, but that also did not work. Is there any reason this might be happening?

Edit: I have also tried to do the same using the installed Startup Disk Creator as well, but when booting the USB option appears blank so I still cannot boot anything other than Ubuntu 11.10.

Can we safely assume the above is a typo? Obviously, you won't be able to install Ubuntu to a 500MB drive. Are you formatting an external 500GB hard drive with FAT32 to install Ubuntu? Don't think that would be good. Do you have unetbootin on the 11.10 partition along with the 10.04 iso file? Where does your hard drive not show up? Do you mean you can see it when you open unetbootin? Can you navigate your directories when you click the tab to the far right of Diskimage?

More details on exactly what steps you have taken and where the problem occurs.

You were right it was 500GB, my mistake. The Ubuntu 11.10 is installed completely to my computer's internal hard drive, and I have completely formatted the external drive so there is nothing on it whatsoever. I can navigate to the drive, it shows up under devices, but when I boot up unetbootin I cannot choose the drive as an option. My goal is to use the USB drive to install 10.04 over my existing 11.10 on the computer's hard drive.

Actually, you cannot boot from an external hard drive.... I've tried it so many times.

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I really don't understand why you are trying to use the external 500GB HD. You used a flash drive to to install 11.10 and you no longer want 11.10 so why not use the same flash to install 10.04 since you have already done it successfully?
In unetbootin, do you have USB in the Type box?
In the Drive box, does anything show?
Do you have a partition on the external drive?
Have you selected the check box Show All Drive (Use With Care)?
I think the problem could be that you do not have a partition on the external but I've only used unetbootin on flash drive so I'm not really sure. That would be the simplest thing to do, create a partition on the external and try again.

If the computer can boot to a usb drive of any kind then you should be able to easily install ubuntu to it.

Just boot to the 10.10 cd and install it to the usb drive just as if it were any other drive. Be very careful on how you do this. To be safe do this first. Remove power from system, remove power to any internal hard drives, plug in usb drive. Power up to bios. Take note of how the drive show in bios. You may have to reboot twice.

Then boot to cd and install to the usb drive. Format it ext or partition it to some smaller amount and leave the rest for fat32/ntfs formats.