Big Calibre transfer jobs can get quite intensive
. I frequently see 'Not Responding' on XP. That's probably because it is just
too busy to get back to the GUI to keep the message away.
As long as the Task/process managers report Calibre CPU (or I/O) usage, just wait
it out. (Keeping a GUI churning wastes CPU cycles on low power systems or sucks up extra Battery power on laptops)
There are tips for Windows users to reduce Anti-Virus scanning of (book) files. MAC's may have a similar setting.