My steps:
1. Restarted in debug mode.
2. Switched to 'all books.'
3. Opened the task manager and put the window on top.
4. Clicked my 'has extra files' saved search.
Observations:
1. Disk usage on my library disk went to 100%.
2. No noticable increase of CPU usage.
3. The search completed after about 10-15 seconds.
The debug log didn't show anything really, just all the usual startup messages.
I repeated the steps except not in debug mode. Same results. I did reboot overnight, so maybe something else was interfering and causing the freeze.