systemd; External hard drive time-out problem during boot.

If you have an external hard drive that is plugged in all the time but on one fine day you boot your system without the external hard  drive and  your system drops to a maintenance shell after looking through the syslog i.e journald  you find out your system got into a loop trying to mount the external hard disk. So what is the solution?
It lies in  the fstab file. In the /dev/sdb line add

nofail,x-systemd.device-timeout=1ms

and do not set it to zero it will go into infinte loop.

for more info check:
https://wiki.archlinux.org/index.php/fstab#External_devices