Could anyone tell me why Scrivener mounts as a device when I start it up? It shows up in the sidebar of the Finder the way the disk image does when first installing it.
Not a problem. In fact, I usually don’t notice it until I drop out of Scrivener to access files in the Finder, sometimes not till I go to shut it down. I assume it shouldn’t be happening, though.
Shouldn’t be happening, you are correct. I would guess that when you installed Scrivener, you might not have actually fully installed it? In other words, you dragged a copy of Scrivener from the installation .dmg to the Dock. OS X is pretty savvy about finding the original .dmg that an alias points to, and automatically mounting it. So if you save an alias to a software on a .dmg and click on it, it will mount the .dmg (which will show up like a disk in the Finder sidebar) and then run the application off of it.
To solve this problem, quit Scrivener and click on the disk image in the Finder sidebar, drag that icon to the Applications shortcut in the sidebar and let that finish copying. Now eject the .dmg, drag the Scrivener icon you created out of the Dock, go to Applications, and drag it back into the Dock from there.
If that is what was causing the problem, then you should see it go away. The only thing I can think of is that you somehow accidentally got the .dmg added to your login items list. You can check that in System Preferences:Accounts.