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重新格式化U盘后仍然如此，由于此U盘之前用于制作Dos的启动盘，怀疑是有隐藏分区之类的。网上搜索了一下，解决办法为 管理员身份运行command prompt
然后依次执行如下指令即可解决问题：
diskpart list disk select disk # clean create partition primary select partition 1 active format quick fs=fat32 assign exit