Will Ubuntu run on a PC with no hdd?

[asifnaz]

Yes you can run live CD or install it on a USB

[Mark Phelps]

... I don't know which of these changes made the difference, but this time Ubuntu booted up without any problems, and everything works! Problem resolved!

Given that you've used this PC for years, it's possible that the video chipset does not work with 12.10 but does work with 12.04. That's because 12.10 uses a new version of the X-server that is incompatible with many of the older video chipset drivers.

[DuckHook]

I have been often accused for making things seem over complex.

You too?!

I guess I don't know what persistence is.

In the interest of keeping explanations simple, I will try to keep it non-technical.

After Ubuntu implemented their LiveCD, someone came up with the idea to do the same thing to a USB stick. By simply writing the disk image that is used for the LiveCD onto a USB stick, you get the same functionality as a LiveCD. But the LiveCD image is designed for a medium that is read only. So, the programs and OS on the LiveCD image have been modified to never write to the disk. You can still download apps and data while running from a LiveCD, but such data resides only in RAM and are lost at shutdown (ergo: not persistent). However, a USB stick is not only writable, but has way more capacity than a CD. It was only natural for some smart geeky sort to modify the LiveCD install so that the OS image is written to one USB partition (which remains read only), but to create and use another USB partition for programs and data. Consequently, data is now no longer just in RAM, but instead is written to your USB stick (ergo: it is persistent).

Charitably, it's sort of a hybrid between a full install and a LiveCD. The OS portion of the stick isn't permitted to change, but another section has been created that can be changed. Less charitably, this could be considered a kludge. And some of us don't like kludges when a cleaner and more elegant solution can be used.

Do I need to reinstall Ubuntu to the stick?

This depends on what you want to do. If your current setup is working well and not giving you problems, then, in the interest of keeping things simple, I would suggest that you not monkey with it and leave things be. However, if you are curious about how to make a full install (i.e. a fully functioning OS that can be modified) to a USB stick, either because you want more functionality or are just curious, then I would suggest the following:

1. Use a new USB stick. They're so cheap that it makes no sense to risk your current setup.
2. Follow @oldfred's instructions here.

[C.S.Cameron]

... But the LiveCD image is designed for a medium that is read only. So, the programs and OS on the LiveCD image have been modified to never write to the disk...

Just a small note:

If you have an ext3 partition named casper-rw on your hard drive,
and while booting the Live CD you press f6,
then type <space> persistent:

 persistent

You will end up with a persistent session with stuff saved to the casper-rw partition on the hard drive.

[honeybear]

I have a Dell Inspiron 5100 notebook with 1.25GB RAM. It worked for years, then got a "Drive 0 not found" error. Replacing the HD did not fix it. Finding that Ubuntu would run from a USB stick, I created one using 12.10 as instructed on Windows and changed my bios order to boot from USB. I used a new 16GB Micro Center jump drive formated FAT32. The notebook now boots to the Ubuntu welcome screen OK, and
everything but the HD works. But when I click on Try Ubuntu, all I get is the desktop picture and a mouse pointer. Will Ubuntu run from USB on a PC with no hdd?

sure. it is regularly done on a Sheeva type PC. en.wikipedia.org/wiki/SheevaPlug

If you have a fast CPU, you can handle Ubuntu.
If not, you could rather look into : http://www.debian.org/CD/live/