Originally Posted by Lady Fitzgerald
Rebooting restored the missing content but I had to reselect which columns I wanted displayed.
You don't need to reboot. You can restore columns by right-clicking on any column heading and selecting 'show', then the column. Selecting 'Restore default layout' will bring all the columns back if, for some reason, unhiding columns doesn't work (which has happened to me).
Switching between libraries without also switching your preferences is asking for difficulties. There are several configuration items besides column layout that might not make the transition easily. For example, saved searches might depend on a column that is no longer there. The saved sort might sort a column that isn't there, or sort the wrong column. Saved user categories might reference items that no longer exist. Tag browser hidden categories might not make sense with the new library, referring to categories that no longer exist. The geometry (shape) of the window might not make sense for the new window. Any of these problems could cause calibre to die mysteriously.
As for using the welcome wizard, as far as I can tell from looking at the code, the fact that it 'works' is an accident. I can't find anywhere that it deals with the problems discussed in the previous paragraph. Because it isn't intended for the purpose you are using it for, changes to calibre could easily make it no longer work as you wish.
To put all of this in perspective and to avoid the 'geek' label (which to me is mildly pejorative), consider a car. A car may actually work with one wheel missing. Of course, it might depend on the weight and placement of passengers, or on never turning toward the side with the missing wheel. But hey, it works with me driving and going in clockwise circles. Or consider a radial arm saw with a dado attachment that isn't tight. It might work with pine, but try it on oak and the blade could fly apart, possibly with disastrous consequences. A more prosaic case might be using a screwdriver as a chisel. It works, until the handle or shaft cracks because of the shocks.
In all three cases, the tool isn't being used as designed. It might work in a particular situation, but that is accidental. The same is true for calibre. Some things might 'work' in the sense that calibre does something, but if that something is not part of calibre's design, then it might not always do the same something. It also might be doing other things that you don't immediately see, and that you would rather it doesn't do.