Comment 10 for bug 574468

I read all the hard steps needed to install Ubuntu server from USB stick but the fix is actually very simple:

Just copy the file "fs-secondary-modules-2.6.32-21-generic-di_2.6.32-21.32_amd64.ude" in the same directory that is located but with the name "fs-secondary-modules-2.6.32-21-generic-di_2.6.32-21.32_amd64.udeb". This step is done on your USB stick after it has been prepared with Unetbootin or other similar tool. The directory that the file is located is "pool/main/l/linux".

The version number of the file and the amd64 part depends on the version of Ubuntu that you had downloaded.

The credits for this fix go to Uniqueone user from the Ubuntu forums. Here is his post:
http://ubuntuforums.org/showpost.php?p=9637343&postcount=4
which I used to create this step. My one is a bit different with coping the file instead of renaming it, because if you rename it permanently in the ISO, you will not be able to install from CD.