Glad that fixed it!
The folder data is stored in a file called 'npf' in /var/hack. (I did have it in /var/tmp, but it looks like that directory gets cleared when your TiVo reboots
The file stores a list of lists. The first entry of each sub-list is the folder name. Then there are zero or more fsids for programmes you've assigned to that folder.
Possible future enhancements that've come to mind so far:
* Sort the folder list - at the moment it stays in whatever order you create the folders. Into alphabetic order would be the easiest option
* Auto-assign new recordings to folders - maybe by series or keyword or a combination of both... not sure yet. Ideas anyone?
I notice TiVo have gone for a grouping option on the new kit they showed at CES. Maybe that's a possibility too...