Open Terminal: Finder > Applications > Utilities
In Terminal, type or paste the following...
(return) (enter Mac password) (return)
A dynamic list of all the processes that are actively reading and writing to the Mac's disk will be displayed as a live stream.
::TIP:: If you see a ktrace_start: Resource busy warning in Terminal then you will have to restart the Mac and run the terminal command again for it to work. This issue may occur if you run the command a second time before the Mac has been rebooted.
Originally posted 10 October, 2021.