For those of us who have a million external runtimes, it is always wise to backup your library.xml file before updating. Make a copy of that file and move it to a holding folder (directory). Use it after updating and if the updating has changed your library.

(If your update has gone smoothly and your library looks okay, you don't need to do anything else.)

The reason why the library disappears is most likely because of this scenario:

During update installation, the screen asks if you want to retain existing preferences, make a backup, or do a fresh installation. Your choice at this point determines your updated library's destiny.

Also, during update installation, the screen asks where you want to install the Poser Content runtime. Your answer here will also determine the destiny of your updated library.

Eternal Hobbyist