Formatting a flash drive that has EFI using Windows 7.

Start -> Run, type DISKPART

  DISKPART> list disk

Find the disk you want to reformat, and select it with the appropriate number,
for example:

  DISKPART> select disk 1

Once selected, you can delete the GPT / EPI by the clean command:

  DISKPART> clean

When complete, you can proceed to format the disk the traditional way through
disk management and selecting FAT32 or NTFS.