Please note that:
- Some BIOS will refuse to load devices without a boot option on a contained partition.
- Some USB devices come with a totally screwed partition table, and the BIOS could be not able to see the partitions properly, and report that not valid USB device is found, even then the MBR actually has a valid boot program.
My advice is to totally wipe the USB disk partition table (that will delete all the USB disk contents), create a new partition table with a new partition, put the boot option on it, and then create the USB Ubuntu again.
- Boot up a Ubuntu Live CD.
- Insert the USB device.
gparted from the Unity dash.
- Choose the USB device on the upper right corner. You'll see then the USB partitions. Please make sure this is the USB device by checking the size.
- Select Device » Create new partition table. If you receive a warning about active partitions, look for partitions with a icon of a key on them, right click, Umount, and then try again.
- Press the Apply button
- Right click on the unallocated space, New.
- Choose the filesystem fat32, assuming this is a standard size USB pendrive (<=4GB), or NTFS in other case. Click add.
- Right clicc on your new partition, choose Manage flags, ensure boot is on and click Close.
- Select Edit » Apply all operations.
Then, wait for Gparted to do its job, close it and make the USB disk again.