Sometimes my disk light comes on when there are no programs running that would be doing disk writes.
I tried htop, but nothing stood out.
Is there a way to find out what is causing the disk activity?
You could try iotop
sudo apt install iotop
sudo iotop
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Sometimes it's normal 
Your OS need to refresh some data - get it from the disk to load it to memory, write logs, ..., background processes also needs to access the disk for read and write. This will be the reason why you see the disk light blinking.
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