iMovie no longer importing analog source correctly -- what's wrong?

Hi all,

I'm using iMovie '11 with a VCR deck connected via RCA to a ADVC-100 Canopus converter that is connected to my iMac via Firewire 800. This setup has served me well for many years, though I've recently taken on a big project to capture about 30 old VHS tapes.The first five were successful, but suddenly I am unable to import correctly.

The image from the VCR plays as it should in the capture window, and when I hit import, it goes into a record mode. However, when I'm done importing, no new clip is made in the Clip Library, and when I go to the finder, the file is there but it has no data (it's reported as being Zero bytes.)

I have tried restarting the iMac, popping in a different VHS. I'm normally pretty savy with troubleshooting these kind of problems, but this has me completly stumped.

Any ideas?

Thanks.