How erase USB stick image ?

[citi]

I finished to install MeteoHub. Now I would like to format my 4GB USB.

But on Windows 7 I can't access to my USB to format it. I think it's because it is a disk image.
So how erase this image ?
thank you.

[wfpost]

you need to use the command tool DISKPART
Windows Vista and W7 do not allow diskmanager manipulating USB storage.

http://support.microsoft.com/kb/300415/en-us/

Be careful to select the correct disk, because all commands with diskpart are executed when you press the enter key!
to erase the whole usb stick you need

clean [all]
Use the clean command to remove partition or volume formatting from the current in-focus disk by zeroing sectors. By default, only the MBR or GPT partitioning information and any hidden sector information on MBR disks is overwritten. If you specify the all parameter, each and every sector can be zeroed, and all data that is contained on the drive can be deleted.

[citi]

the problem when I run DISKPART and "list disk" I don't see my USB, only the normal c:\\

I see my USB only in windows, setup, periheral and printer. In the property I have the name of the USB : Crucial Gizmo! USB Device.