Does a free app exist that can tell you WHAT is writing to the hard drive?

Does a free app exist that can tell you WHAT is writing to the hard drive?

Here's the situation that has been happening for months:
1. I'm very used to Veracrypt where there is a setting to have it unmount a
mounted encrypted drive when nothing has been written to it for a given
time period (usually around 30 minutes or so is the default).
2. On _other_ computers, this works just fine to automagically unmount the
encrypted drive after nothing has been written to it for the stated time
period.
3. But on _one_ of my devices, even when I walk away from the computer for
longer than the stated period, most of the time (but not all the time), the
encrypted drive remains mounted. Even overnight, it remains mounted (most
of the time).

Hmmmmmm....

Is something _writing_ to the encrypted drive that I don't know about?
o How would I figure that out?

NOTE: I don't know of anything I would have set up that would do that,
e.g., I don't have keep-alive programs running, nor do I run indexers (at
least not on purpose), nor do I have automagic backups, etc., that I know
about.

Obviously there could be _something_ that I don't know about that is
writing to the drive ... but how would I find that out?

It's not a "big deal" but it's an enigma to me.
o Does a free app exist that can tell you WHAT is writing to the hard drive?