Step1:下載img文件
FreeBSD-12.1-RELEASE-amd64-memstick.img
Step2:插入U盤，識別並准備
Step3:修改系統，允許往U盤寫入引導
root@FreeBSD:/data # sysctl kern.geom.debugflags=16
kern.geom.debugflags: 0 -> 16
Step4:寫入鏡像到U盤
root@FreeBSD:/data # dd if=FreeBSD-12.1-RELEASE-amd64-memstick.img of=/dev/da1 bs=64k
