Thanks for the explanations! This is the only thread that helped me understand why my CHM Help file Favorites cannot be backed up, and are difficult to even find.

In my case I had a bunch of Favorites from AutoIt CHM help file that got corrupted and had to be painfully recreated. I then tried to back up the new Favorites list but there is no option. The Help CHM file itself never changed Size, no matter how many Favorites were added. Where was all that extra info stored ???

I see now the only solution to back up Favorites it to copy the entire hh.dat file, but a lot of the info in there is not relevant. HH.dat seems to hold ALL the Favorites of every CHM Help file for that user on that computer. At least now I can do that. Again, thanks.