Do either the corrupt PZ3 or the backup PZ3 open correctly after Poser has been restarted and/or the system has been rebooted? If you are using P4 or ProPack, this may be an instance in which deleting the .rsr files might help. Sometimes the .rsr files in Geometries may become corrupted. You might try deleting the .rsr files associated with any geometries used by your problem file. Try opening up the .PZ3 in a text editor to see if any information was saved. That will tell you whether the problem is the .PZ3 or Poser itself.... I have had the sort of problem you relate happen periodically with both P4 and P5. The only remedy I know is to save regularly as you work. It might help something to make the Poser.rsr read only, although I'm not really sure how that is supposed to help. Hopefully someone else can elaborate....

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Cage can be an opinionated jerk who posts without thinking.  He apologizes for this.  He's honestly not trying to be a turkeyhead.

Cage had some freebies, compatible with Poser 11 and below.  His Python scripts were saved at archive.org, along with the rest of the Morphography site, where they were hosted.