I think I've managed to blow up a nice piece of software. My fiction library is in the neighborhood of 46k files and calibre managed to get as far as H/_Misc/ before hanging on the first pass (before actually importing).
The obvious work around would seem to be adding files in slightly smaller increments.
We won't even talk about the non-fiction portion of the library. I don't have a clue how many files are in that can of worms.
Anyhow, just thought I'd drop a note. I don't know if this is a known issue or just my own problem. OS is linux 2.6.32-22 under ubuntu 10.04. The behavior is a hang - no crash, no loop. Processes drop into sleep mode and hang out indefinitely. The import of Fiction/A seems to be going OK so far.
If you need/want any more info, just ask.
Oh, calibre version is 0.6.42. Maybe this is already fixed!