iotop might or might not need to be installed depending on your Linux distro, I’d expect it is in most standard repositories.
lsof was a new one to me but seemed worth suggesting.
As far as I know Gnome System Monitor sums the disk reads/write across all drives. I mainly run my systems from USB drives and it certainly shows those. So my idea for this is for you to see which process is busy reading or writing.
FWIW from some other experiences I can attest that drives can spend amounts of time looking active (light flashing) even though the computer thinks nothing is happening. I found I had acquired a couple of SMR technology drives and they will routinely spend up to 30 seconds rearranging data internally - and put the computer I/O into a halt state for the duration. Maddeningly, every performance test I ran just ignored these delays in their metrics and gave the drives a good score. Lookup SMR vs CMR at your peril.