Hi,
I have found the reason, unfortunately as Windows do not supply Drive partition S/N in API, on start MM needs to open and read all available drives (those with drive letters) to get Partition S/N and pair it with MM library. After start MM relies on Windows Drive Change Event to repeat same on reported change.

This unfortunately can't be disabled.

EDIT: This is same as of MM 1.x

No, no and no. I've now tried backing up then removing my database and ini settings, to no difference. Cutting out all default plugins from the program folder didn't help. Even a clean, portable installation of the latest MediaMonkey 4 seems to do it. At this point I'm giving up and assuming that checking all available drives on launch is just baked into the code somewhere on account of being irrelevant to the typical user. (That or Windows itself is being overzealous.)

So no Windows or other software is running on the drive? No reference to it under the Entire Library > Location (enable under File > Manage Collections) to it either?

Are you browsing through the My Computer node (or is it expanded) in the Media Tree?

Video games and mixed media storage. Occasionally I'll play audio from the HDD (eg. checking out a game's soundtrack) but nothing in MM's current library is on there.

What's on the hard drive?

Recently I've been trying to quieten my computer when not under heavy load, mostly for the sake of listening to music. All of my music files are stored on the same SSD that MediaMonkey is installed to and the library is only monitoring one folder on that SSD. Annoyingly though, MM still hangs briefly / wakes up my somewhat-noisy HDD that would otherwise be turned off by Windows power settings when not in use. This happens when MM is first launched but not when actually in use or if it's closed and reopened later. Is there an option or plugin I might have overlooked that could be causing this?

Full disclosure: I'm actually using MediaMonkey 3 since I didn't care much for 4's UI or need its extra features. If this problem doesn't occur in modern builds then, uh ... sorry and please let me know I guess?