mkdir -p ~/.local/share/cinnamon/applets/sd-activity@rob

~/.local/share/cinnamon/applets/sd-activity@rob/metadata.json

~/.local/share/cinnamon/applets/sd-activity@rob/applet.js

once the files are in place, you can install the applet by following these steps:

cinnamon --replace &

The screen may flicker briefly.

After that:

Right-click the Cinnamon panel. Choose Applets. Find SD Activity. Add it to the panel.

It should show:

dark gray while idle green on SD reads red on SD writes yellow when both counters change together nearly black if mmcblk0 is unavailable