Hard drive volume appears in sidebar Favorites when new disk mounts

Question

Hard drive on my 17” MacBook Pro Quad Core i7 (2.5 GHz, 8GB SDRAM OS 10.8.2) is partitioned into 4 volumes; I'll call them A-Mountain Lion startup drive, B-Personal, C-Work, D-minimal clean Mt. Lion for emergencies. Problem relates to my regular boot drive.
I have Finder sidebars set to show 8 or 10 folders in the "Favorites" section and have ticked all boxes in the "Devices" section. Most of the time I have no external disks, CDs, etc., connected to the MBP, so the Devices section generally shows my computer and the four partitions. So far, so good.
Lately, every time an external volume is mounted and every time a disk image is mounted, the Personal volume (B) of my internal drive, which is already showing under Devices, also pops up in the Favorites section of the sidebar. I remove it from the Favorites, but the next time an external disk or a disk image mounts, there it is again. It's always volume B, never A, C, or D.
I have tried everything I can think of to cure this odd behavior: Used Disk Utility (verify all disks/volumes; repair volumes; repair permissions; etc.), though nothing seemed wrong. I've run Disk Warrior on all volumes. I cleaned out everything possible on the startup drive (A) with the latest version of Onyx. I've repeatedly trashed the plists I could identify in my user Library that had "sidebar" or "finder" in their names. I've logged in and out; started and restarted; started with shift key down. Still, every time a new volume or disk mounts, up pops this extra volume B among the Favorites (where it shouldn't be). I checked and unchecked various combinations of sidebar items in Finder Preferences. I've Forced Quit the Finder. It doesn't matter whether all, some, or no devices are checked; when a new disk mounts, there's the extra volume B again.
I have only two other clues to add. (1) I created a new temporary user account on my startup drive (volume A), and this behavior did NOT occur in the new account. (2) This same behavior had arisen once before, a couple of months ago. I went through similar blind attempts to cure it then, but at some point I must have done something right because I it got fixed. For the life of me, I can't remember which of all the things I tried did the trick.
Many thanks to anyone who can shed light on this annoying behavior.

Answer 1

I can add a bit to my earlier description. I had said the problem did not occur in a new user account. Well, after using the new account for a while, it showed up. Eventually I erased the startup volume and installed Mountain Lion (plus updates to 10.8.2) fresh. I used migration assistant to transfer only my old user, pared down applications, and network settings -- no preferences, application support stuff. The old user carried over the problem. But I immediately created a new user and have been working from it for the past two weeks, very gradually opening apps, adjusting settings, etc., and testing for the anomaly at each change. In some cases I did move preferences from a backup clone, but was always careful to test after each addition. All was well for a week, until one morning I noticed the data partition had appeared in sidebar Favorites. So I'm back to where I was. I had been documenting each change or introduction I made. The only things my notes said from the day before the sidebar anomaly appeared was launching and struggling to activate QuarkXPress 9.3, and copying over my old prefs and settings for TextWrangler. I completely removed every trace of QXP, but the problem remained. (I did not remove TextWrangler because I just can't believe it could be the culprit -- but could it?)
I also noticed a similar (though maybe not identical) problem posted in another thread <https://discussions.apple.com/message/20591050#20591050>, but so far no answers there either.