This was the solution for me! Thanks.
Edit.. I was too soon with my answer. This wasn't it. For me the problem seems to be solved after accessing the disk in Unraid and clicking through the tabs:
It seems that after doing this, the drives don't start up anymore. If this is really the solution, then this is definitely a bug in Unraid and should be patched.
It is now 17:00 and the disk hasn't spun up yet, so this seems to be it...?
I also tried the following to solve it, but to no avail:
move docker .img to SSD
clean up shares
play around with Tunable (poll_attributes) values
remove Unassigned Devices plugin (and reinstall after concluding it didn't have any effect)
sign out of unraid server plugin
remove NERD Pack plugin
install scrutiny docker app
stop docker entirely
stop part of some docker apps (i.e. Plex)
stopping all cron jobs
disabling SMB on the shares that use the drives that are constantly spinning up
Hope this helps some of you. Again: if this really is the case, then please devs, fix it!
Edit 20-aug, the saga continues....
Look like I finally find the culprit. My Unraid is running as a VM in ESXI. ESXI uses smart daemon every 30 minutes.
After disabling the smart daemon using command:
/etc/init.d/smartd stop
the polling stopped! Problem solved!