I'm trying to think of anything I may have changed since the last time I rebooted the opnsense VM. But I try to keep up on updates and end up rebooting pretty regularly. The only things on this system are the opnsense VM and a small pihole VM. At the time of the screenshot above, the opnsense VM was the only thing running.
If it's not a failing HDD, my next step is to try and dig into what's generating the I/O to see if there's something misbehaving.