First, install arch-install-scripts:
sudo pacman -S --needed arch-install-scripts
Secondly, mount your partitions in all the internal hard drives.
Thirdly, generate and validate your config by piping it out to stdout:
genfstab -U -p / | less
If all looks green, proceed to overwrite your current fstab with the new entries:
genfstab -U -p / >> /etc/fstab
Run that command as ROOT.
Tip: It may be wise to add the
nofail option for some partitions, namely NFS and the likes, so your system can boot even if shit hits the fan.