I run this file in a cron job every 5 minutes. It checks whether your drives' usage has exceeded a configurable threashold, and if so, it sends an email to a configurable e-mail address.

disk_space_checker.py

#!/usr/bin/env python3
"""
A simple program that sends warnings when disk space on one or more hard
drives has reached a critical level
"""
from subprocess import Popen, PIPE

##############################################################
#CHANGE SETTINGS BELOW
##############################################################
EMAIL_ADDRESS = 'davidgrant@gmail.com'
THRESHOLD = 90
#DRIVE_NAMES = ['/dev/hde1', '/dev/hde2']
DRIVE_NAMES = []
EXCEPTIONS = ['sr0', 'sr1', 'none', 'udev']
#EXCEPTIONS = []
#if SEND_EMAIL=True, email will be sent, else, just stdout
SEND_EMAIL = True
USE_SUDO = True

##############################################################
#DON'T TOUCH ANYTHING BELOW UNLESS YOU KNOW WHAT YOU ARE DOING
##############################################################

def main():
    """ Main method """
    msg = []
    cmd = ['/bin/df',
           '--exclude-type=udf',
           '--exclude-type=fuse.gvfs-fuse-daemon',
           '--exclude-type=tmpfs',
           '--exclude-type=devtmpfs',
           '--local']
    if USE_SUDO:
        cmd = ['sudo'] + cmd
    output = Popen(cmd,
                   stdout=PIPE, shell=False).communicate()[0].decode()
    lines = output.split('\n')[1:-1]
    drives_map = dict([(line.split()[0], int(line.split()[4][:-1]))
        for line in lines])
    # Get included drives or all
    drives = [drive for drive in drives_map if (drive in
            DRIVE_NAMES or len(DRIVE_NAMES) == 0)]
    # Exclude some drives
    for exception in EXCEPTIONS:
        drives = [drive for drive in drives if not exception in
                drive]
    # Check which drives exceed the percentage
    drives_exceed = [drive for drive in drives if drives_map[drive] >
            THRESHOLD]
    # Make an email message
    for drive in drives_exceed:
        msg.append("drive " + drive + " is too full.")
    if len(msg) > 0 :
        msg = "\n".join(msg)
        if SEND_EMAIL:
            mail_command = 'echo "%s" |mail -s "[disk space full]" %s'
            Popen(mail_command % (msg, EMAIL_ADDRESS), shell=True)
        else:
            print(msg)

if __name__ == "__main__":
    main()