Why is Windows constantly spinning up my secondary HDD?
Base
Process Explorer has a special column that tracks the change of the number of bytes read or written by a process:
You might be able to capture the name of the process that is reading or writing data to your secondary HDD this way.
//Edit:
If you know that it is the System process that writes something to the external hard drive, you could trace what the process is doing in the Process Monitor:
Start/stop the capture with Ctrl+E.
Define some of those filters:
//Edit2:
Maybe you could track the thread ID associated with a certain IRP event and then track the thread that caused that IRP event:
SOLVED!
I stopped being fancy and decided to go in and disable all running services one by one until the computer crashed or my disk stayed idle. Luckily, I didn't need to go all the way to a crash and found the service "Sentinel LDK License Manager Service" to be responsible for the wake ups. I googled it and it seems to be related to Lightworks which I then uninstalled. The Sentinel Service did not uninstall with Lightworks, but somehow my HDD stays idle now that Lightworks is gone.
So, I guess the problem was somewhere there... Lightworks and the sentinel service did not get along and this caused my secondary HDD to wake up through writes in parts of it's "unpartionable" space... yeah that doesn't make much sense to me, but if someone googles this, you know what to do.