Ubuntu images (and potentially some other related GNU/Linux distributions) have a peculiar format that allows the image to boot without any further modification from both CDs and USB drives.
A consequence of this enhancement is that some programs, like parted get confused about the drive's format and partition table, printing warnings such as:
/dev/xxx contains GPT signatures, indicating that it has a GPT table. However, it does not have a valid fake msdos partition table, as it should. Perhaps it was corrupted -- possibly by a program that doesn't understand GPT partition tables. Or perhaps you deleted the GPT table, and are now using an msdos partition table. Is this a GPT partition table? Both the primary and backup GPT tables are corrupt. Try making a fresh table, and using Parted's rescue feature to recover partitions.
Warning: The driver descriptor says the physical block size is 2048 bytes, but Linux says it is 512 bytes.
All these warnings are safe to ignore, and your drive should be able to boot without any problems.
Refer to the following message from Ubuntu's mailing list
if you want to learn more.