Update on this whole topic.
Although I am a TWC/LA customer, using Motorola's latest DCX3400 (cousin to their 7100 model for FIOS), I and many other AVS Forum members have reported experiencing the exact same firewire-recording failures as you've so accurately described on the 7100 for FIOS. I mentioned this in my earlier reply.
However I have gotten the ear (and attention) of the Director of Video Platforms at TWC/LA, and he's accepted my offer to come to his office with my JVC 40K and DT100 VCRs to demonstrate the "firewire failure" to him and and an engineer who works for him, using a DCX3400 in his office. I'm going there this Monday, having requested that they also have available an older DCH3416 and/or DCT6416 so that I can demonstrate "firewire success" using these older model DVRs.
In turn, they have an onsite visit from Motorola engineers planned for later in the week for other reasons, but this whole topic of the firewire issue will be brought up. Hopefully they will be able to repeat my demonstration of the firewire anomalies to Motorola that I will hopefully be able to demonstrate to TWC on Monday.
Anyway, as part of the preparation for Monday's visit to TWC, I was sent some screenshots that Motorola sent to TWC ostensibly "proving" that firewire recording from DCX3400 to DVHS (a 40K, in Motorola's lab) actually WORKS!
Well, it turns out that their screenshots of "success" were just for 480i SD recording (at STD speed of 14.1Mbps). They did not also provide "HD proof" with screenshots of a similarly successful 720p/1080i HD experiment (at HS speed of 28.2Mbps). Now I don't know if Motorola actually has performed those HS experiments as well and just neglected to send them, or if they actually have never performed HD tests but simply thought their SD "success" proved that nothing was broken.
Anyway, I've asked TWC to request Motorola to perform a similar experiment as before but this time using HD 720p/1080i content... and then report as to success or failure.
And here, in this forum (as well as over on the dedicated DCX3400 thread
where this issue has been discussed for many months), I've asked anybody (such as yourself) who's previously observed problems recording via firewire from either the DCX3400 for TWC/Comcast or the 7100 for FIOS, to try and replicate the Motorola results... of SUCCESS recording SD 480i content via firewire. You will either also have success, or you will have failure... and I'd like to know which it is.
Of course if you no longer have a 7100 (i.e. you've returned it for an older 6200 so that you can have your firewire recording back) then you can't really help. But hopefully if there are other FIOS 7100 users out there who can contribute to this 480i firewire experiment, I'd appreciate it.
If your 480i recording results are a success, but your 720p/1080i results are still failure, that's very significant information that Motorola needs to know about.
But if your 480i recording results are still a failure, as are your 720p/1080i results, then Motorola needs to review their experimental lab testbed environment to explain why their results lab differ from our results "in the wild" for the identical experiment.
Of course if Motorola's 720p/1080i test fails, and your 480i test works, then that's the ultimate "good news" for all of us. They've finally been shown that there is a failure, and also with a "giant clue" that it's somehow SD vs. HD bitrate related on the DCX3400/7100 models, and probably not related to the 2-port vs. 1-port dropdown.
Please... try the 480i firewire recording experiment on DCX3400 and 7100 FIOS boxes, and report results here.