hddtemp is a small utility (daemonizable) that gives you the temperature of your hard drive by reading S.M.A.R.T. information (for drives that support this feature).
Sync and install with pacman:
# pacman -S hddtemp
Hddtemp can be invoked with the drive as the argument:
# hddtemp /dev/sd[x]
Where x is the drive.
Running the daemon gives you the possibility to access the temperature via an TCP/IP request, so you could use this in order to check the temperature from outside, or within some scripts.
Once hddtemp has been installed, standard systemctl procedures work to start the daemon:
# systemctl start hddtemp
To start it on boot, enable it:
# systemctl enable hddtemp
Another way to get the temperature is by connecting to the daemon which is listening on port 7634.
$ telnet localhost 7634
Or with netcat:
$ nc localhost 7634
Refer to the manpage for information like supported drives, logging, etc.
$ man hddtemp
Hddtemp can be easily integrated with system monitors like Conky, the for GNOME Panel or the for Xfce Panel.