I'm accustomed to having T'bird ask if I would like to compact folders, saying it would save 20+ Mbytes to do so. In the past two or three days, however, after I say "Yes", the compacting process slows to a crawl, T'bird becomes unresponsive, and then pops up an error message that "There is not sufficient disk space available to complete the operation" [that's the gist of it as I remember, not copied directly from the screen.] Eventually a different message reports that compaction was unsuccessful owing to insufficient space available.

  My hard disk has more than 100 GB of space free.   What/where is the size-limit set that prevents compaction from finishing?  I have tried clearing the cache, also increased cache size to 1024 Mb.