What goes wrong in my eyes: The files shouldnt be read while browsing the db.
Just making sure because we've discussed it previously, but you have
Tools > Options > Tree & View > Advanced:
Display Missing file image in lists disabled, correct?
Because with that
on, then it does absolutely have to check every file on disk in the visible file list in order to display them.
For what you specified, you'd also want to make sure: (1) all of your thumbnails are pre-built and cached (otherwise it will read the files to build thumbnails), and (2) Auto-Import Settings:
Update for external changes is disabled. For the latter, I'm not positive it would impact just browsing around, but that would at least periodically trigger MC to spin up the drives and read the files. And, if any changes are made to any of the files, then Auto-Import could trigger when the filesystem event happens (it also just triggers on a timer for full-scans as well).
Lastly, you may have already done this, but you may really want to check power draw at the wall before you assume that sleeping the drives actually saves power. A single spin-up of an array can cancel out the power savings for a LONG period of idle-spinning. With my usage patterns, when I tested power-draw at the wall, it did not make sense at all to let my main media array sleep.
For my cold-storage drives, definitely. But I disconnect those from the dock and put them on a shelf when they're not in use, so that doesn't matter in practice. But for my main online media array, it was costing more in power (and drive wear and tear) to let them sleep when I actually tested them, and so I now have them spinning all the time.