Working with a fresh Seagate hard drive in my machine, I am having the same problem with Capture Now locking up. The factor that seems to be causing the trouble is when I capture for more than five minutes. I had this happen twice on a capture longer than that. After forcing the computer to shut down, I looked at my media drive and noticed the clip I had been capturing had expanded to fill up the drive to the limit I set in scratch disks.
Recapturing in pieces under 5 minutes allowed me to continue with no trouble.