The system load indicators included for the panel does not seem to be able to monitor the hard disks (reports 0% use). Processor, network, memory & load average all seem to work fine.
What version of Ubuntu-MATE?
$ lsb_release -a
Distributor ID: Ubuntu
Description: Ubuntu 16.04.1 LTS
Release: 16.04
Codename: xenial
Hi @thumbdrive,
have you run a full system update to see if it fixes the problem?:
I did as you suggested (update via recovery mode & dpkg) and the monitor is still showing 0% for hard disk activity. Also, Gkrellm reports fine.
have you tested initiating an operation you know to be heavy in disk I/O?
That indicator is scaled like the others.
Try something like burning an USB key with a Ubuntu MATE iso or reading a slideshow from a pictures folder on an external HDD.
Cheers
Hi @thumbdrive,
try the following terminal command (Ctrl + Alt + t) and see if that helps:
sudo apt-get update && sudo apt-get remove mate-system-monitor && sudo apt-get install mate-system-monitor
Restart after running the above command!. 
Thank you for your suggestions. I removed and reinstalled mate-system-monitor, restarted and then copied a large file between drives, still showing no activity. Gkrellm is reporting fine.
HI @thumbdrive,
try opening Synaptic (install if necessary) and firstly change your software sources download location, then search for mate-system-monitor and then right click it and mark for re-installation!. 
Terminal install command (Ctrl + Alt + t) as follows:
sudo apt-get install synaptic
See also:
How is this disk setup? Is it a raid? Is it encrypted? What is the device name?
Type the following at your terminal (replacing sda with the affected device name):
$ watch -n 3 grep -w 'sda' /proc/diskstats
Do you see the values changing every 3 seconds?
(use CTRL+C to exit)