linux 下对设备直接读写就可以

那U盘的文件名应该是什么呢 /dev/sdb ?

用 dmesg 看 USB 变成哪一个 sd[x]. (sda, sdb, sdc, sdd, sde)

# sudo dmesg
scsi2 : SCSI emulation for USB Mass Storage devices
  Vendor: Sunplus   Model: MultiMedia-Disk   Rev: 1.00
  Type:   Direct-Access                      ANSI SCSI revision: 02
SCSI device sda: 1974272 512-byte hdwr sectors (1011 MB)
sda: Write Protect is off
sda: Mode Sense: 55 53 42 43
sda: assuming drive cache: write through
SCSI device sda: 1974272 512-byte hdwr sectors (1011 MB)
sda: Write Protect is off
sda: Mode Sense: 55 53 42 43
sda: assuming drive cache: write through
sda: sda1

备份 /dev/sda 的 MBR
# sudo dd if=/dev/sda of=/tmp/mbr.backup bs=512 count=1

直接开 /dev/sda 读写即可 (要用 root 权限或 sudo)

(造 /root/backup 目录)
# sudo mkdir -p /root/backup

(作备份)
# sudo dd if=/dev/sda of=/root/backup/mbr.bin bs=512 count=1

(清空为0, 不能读取)
# sudo dd if=/dev/zero of=/dev/sda bs=512 count=1

(存回去)
# dd if=/root/backup/mbr.bin of=/dev/sda bs=512 count=1

// mbr.c
// gcc -o mbr.exe mbr.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

#define LEN 512

int main()
{
  int fd;
  unsigned char buf[1000];
  int ret;
  int i;

  fd = open("/dev/sda", O_RDWR);
  ret = read(fd, buf, LEN);
  for(i = 0; i < LEN; i++) {
      buf[i] = buf[i] ^ 0x9d; // xor 简易加密, 第二次回复.
  }
  lseek(fd, 0, SEEK_SET);
  write(fd, buf, LEN);
  close(fd);
  return 0;
}

感谢各位 结贴了