I always wonder what is my computer doing when it takes long time to shut down? Incidentally I found a brief good explanation on reddit.
Just in case you're too lazy to click the link, I copied down the answer here:
The computer does many things when you shut down. This post is going to cover as many as I can think of.
One of the simplest and most important has to do with memory. The computer has to insure that anything that needs to be written to disk, is written to disk. If you don't do this, you could end up with corrupt or invalid files. If you want more detail than that, you're going to have to take a few years of computer science at a college level.
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