Not sure if this has been mentioned before but I came across this through Pete Scargill’s blog the other day and it’s quite amazing if you want to see what your system is doing.
Simply search on netdata and then use the instructions to install it (a couple of curl command lines).
One great thing it proved to me was that after moving my system to a USB HDD I am absolutely certain that the SD card is no longer being used. You have to scroll down a bit on the dashboard but there’s no file i/o activity on the card - a lovely flat line.