Step1:下载img文件
FreeBSD-12.1-RELEASE-amd64-memstick.img
Step2:插入U盘，识别并准备
Step3:修改系统，允许往U盘写入引导
root@FreeBSD:/data # sysctl kern.geom.debugflags=16
kern.geom.debugflags: 0 -> 16
Step4:写入镜像到U盘
root@FreeBSD:/data # dd if=FreeBSD-12.1-RELEASE-amd64-memstick.img of=/dev/da1 bs=64k
