Outlook is using around 85% disk (approx 2MB/s). I've only noticed it in the last few days although it could have been going on for longer. The high usage is constant and all the time 24/7 (unless Outlook is not running of course).
If I close and restart Outlook, the disk usage is initially a sensible number, but gradually creeps back up to 85% over the course of around 5 minutes.