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Say this is the case requires a low-level format of the USB flash drive, the following methods:
You can try using the DiskPart command
① run cmd as Administrator
② then enter Diskpart.exe
③list disk//can display all disks, including USB flash drive
④select disk X//select USB drive as the currently selected drive letter, after execution in the Use list Disk command can see the disk corresponding to the drive letter A star, here X for the list disk found in the U-disk number, usually 0,1,2 such a number
⑤clean//Clear the contents of the selected drive letter disk, then you can format the USB flash drive in Disk Management.
⑥ then go to disk Management to see if you can format the disk.
U disk to do a boot disk to install Ubuntu, installed, the U disk can not be formatted, now say the online search method