It could be a metadata conflict that can't be resolved, depending on how you are using Picasa. For some files and certainly for all raw files the information is stored in the Picasa.ini file contained in each folder.
Thanks for your reply. I finally figured out what was giving me all these problems (saving metadata stuck, face detection hung, LR hung). I had about 50 JPG files that seemed to have corrupted metadata. LR wouldn't load them at all (even though they had been in the LR data base). Windows Photoviewer and other applications handled the files just fine and there was really no indication of what was wrong except that they wouldn't show previews, wouldn't re-import. I finally loaded them up in PicMeta Information Extractor (excellent tool) and did a Metadata repair. After that LR loaded them fine. LR should really provide a better indication of what the problem is and how to fix it. It simply hung on those images rather than giving workable error messages.
1 person found this helpful
In addition to that, Lightroom will hang on saving Metadata for selections/folders that have video files in them!!! BUG