Input video stream plays in Media Import window but won’t record

I’m trying to capture video from an old device with S-video out. I've cobbled up a converter and a bunch of adapters to ingest the video stream into my iMac. My goal is to use FCP to record the video stream. Much to my delight, I can see the video stream play in FCP's Media Import window. When I hit the Import button, I see the timer proceed in the upper right of the Import window and I see the Importing (x:xx m) timer proceed. However, when I stop the Import, no file shows up in the FCP browser window or anywhere that I’ve searched. Initially, I was concerned that my cobbled-up hardware was the culprit, but if FCP can play the video stream in the Import window, why can’t it record the stream? I've checked my Import preferences and everything seems to be in order. Any ideas on how to resolve my problem?

The devices I’m using to capture the video are:

• Analog S-video source into a Dazzle A/D converter - output is Firewire 400
• Firewire 400 to 800 adapter
• Firewire 800 to Thunderbolt adapter
• Thunderbolt to USB-C adapter into my iMac