PiLEDlights

For the Raspberry Pi - Make LEDs blink on network and/or SD card/USB storage activity.

hddledPi blinks a LED connected to a GPIO pin on any mass storage access. Not only on SD card access, but also on USB thumbdrive and hard drive activity.
netledPi blinks a LED connected to a GPIO pin when there is activity on any network interface. Not only the built-in ethernet interface, but also on any other USB ethernet or WiFi interface.
actledPi blinks the Pi's ACT led on all mass storage I/O, i.e. not only the SD card.

netledPi and hddledPi make use of Gordon Henderson's wiringPi library - wiringpi.com - so you have to have that installed in order to build the programs. Current versions of Raspbian come with wiringPi already installed.
EDIT 2020-05-19: It seems like the Pi 4B needs WiringPi 2.52. At the time of writing this, Raspbian comes with 2.50, i.e. you'll have to update WiringPi if you are on a Pi 4B. http://wiringpi.com/wiringpi-updated-to-2-52-for-the-raspberry-pi-4b/

Building the programs is easy:
gcc -Wall -O3 -o netledPi netledPi.c -lwiringPi
gcc -Wall -O3 -o hddledPi hddledPi.c -lwiringPi
gcc -Wall -O3 -o actledPi actledPi.c

I recommend that you copy the binaries to the /usr/local/bin directory, because the init scripts provided all assume that they reside there.

hddledPi uses wiringPi pin 10 by default. It is BCM_GPIO 8, physical pin 24 on the Pi's P1 header.
netledPi uses wiringPi pin 11 by default. It is BCM_GPIO 7, physical pin 25 on the Pi's P1 header.
Note: These pins are also used for the SPI interface. If you have SPI add-ons connected, you'll have to use the -p option to change to another, unused pin.

Options for netledPi and hddledPi:
-d, --detach Detach from terminal (become a daemon),
-p, --pin=VALUE GPIO pin (using wiringPi numbering scheme) where LED is connected
-r, --refresh=VALUE Refresh interval (default: 20 ms)

Options for actledPi:
-d, --detach Detach from terminal (become a daemon)
-r, --refresh=VALUE Refresh interval (default: 20 ms)

netledPi and hddledPi need super-user privileges, so you have to start them with "sudo", e.g.
sudo netledPi -d -p 29

I have only tested the programs on Raspbian.