When Directory Opus 13.12 is configured to replace Explorer (as it is now the default), it would open new window with removable drive as the only tab in the left pane (with the default configuration).
However, when Directory Opus or MS Windows crashes, or MS Windows needs to be rebooted - Directory Opus remembers only one of the windows that there were opened, instead of opening both. Thus is forgets all the tabs that were configured / added for the left pane, losing user configuration.
This, in by opinion, is a bug (or at least a misfeature).
Directory Opus should either open both windows as they were last seen, or open one window with the concatenation of unique tabs from all windows opened, for both panes.
Fortunately I had saved configuration when upgrading Directory Opus, otherwise I would have lost all my "bookmarks" to different places in the filesystem, in the form of per-pane tabs.
There's no bug here, just the default configuration assuming that if you open a window for a particular folder you only want to see that folder. You can change that if you want something else.
Set Preferences / Launching Opus / Startup to re-open previous windows.
Set Preferences / Launching Opus / Explorer Replacement to open all Default Lister tabs when opening a new window for a folder.
The bug, in my opinion, is that Directory Opus does not open all windows (listers) that were opened, only the last opened window (lister).
Perhaps going to Preferences / Launching Opus / Startup, and selecting Backup open Listers automatically would help. In my case Directory Opus was not closed cleanly.
If Open a new tab instead of a new window (lister) were the default in Preferences / Launching Opus / Explorer Replacement, I would not have the problem of potentially losing all my saved/configured tabs on [hard] reboot of MS Windows.
You can configure Opus to re-open any previously open windows at system startup, if you want that.
If you don't turn that on, and then launch Opus by asking it to open a particular folder, it will only open that folder (plus other Default Lister tabs, if you configure those, but not other windows).
This is all by design. (And several people have already had problems with old windows piling up because they never close any and keep opening new ones in addition to the old ones, so there's a balance here with re-opening old windows when they weren't what was actually asked for.)
Essentially, if you never want to lose a window except by manually closing it, even across restarts, then you need to set Opus to launch on startup so it can re-open the windows after a reboot.
You can configure Opus to re-open any previously open windows at system startup, if you want that.
I didn't have this configured (it is off by default). Perhaps I should - one of first things that I do after restarting MS Windows is starting Directory Opus.
If you don't turn that on, and then launch Opus by asking it to open a particular folder, it will only open that folder (plus other Default Lister tabs, if you configure those, but not other windows).
I have launched Directory Opus from system toolbar (which should give the same result as launching it from the Start Menu, I think).
It is unfortunate convolution of defaults that led to my problem, namely:
when replacing Explorer, opening removable USB drive in new window, instead of in a new tab
Directory Opus opening only the last window (lister) when run from Start Menu / toolbar
no way to restore tabs from other closed listers (that I could find) - closed by force rebooting MS Windows