If you had a trial of Chrome OS with your USB and after you are done if you find out that your USB disk is showing incorrect size, or can’t be formatted, or has several partitions, here is the way to fix that problem.
Use Gparted, in windows download it from here: http://gparted.sourceforge.net/
In linux, go to terminal, type
sudo apt-get install gpartedand press y.
Select your USB disk from the top right corner, and delete all the partitions one by one. If the delete option is not available, use the Unmount option first, then Delete option will be there.
After all the partitions are deleted and the whole space is shown as “Unallocated” (the color should be grey), right click on the unallocated space and select “New”, format it to NTFS or FAT32 whatever you wish.
Note: The disk management utility of Windows can NOT fix it. This is the easiest method for me.
The same task can be accomplished by using any other partition manager utility.