after importing my refs from pubmed I allways grade and file them in groups - (every single one!). I now noticed that 300 refs are unfiled! I am sure I did file them in one of my 40 groups but this link seems to have been broken. I do file some refs in more than one group - sometimes even 3-4 . I am pessimistic in restoring these links (back-up has the same issue) but how to avoid this mishap in the futue (besides checking for unfiled refs before closing en creating a back up!)