Again, I think Tom's answer on this was satisfactory, in that he stated he doesn't have access to design docs and isn't an engineer. The statement of "cards slow down over time" absolutely makes sense if you understand how flash memory works and physically degrades over time. The cache memory on a HDD isn't as direct an analogy as you may think, because a HDD doesn't physically degrade in the same way as flash memory. A HDD can certainly develop bad sectors, but generally with good disk management the performance of the disk overall isn't going to degrade all that much unless it's an extreme situation. This is not the case with flash memory - when things start wearing out (and they will), you will start to see performance degradation on a more global scale. SSD drives employ "wear leveling" to help combat this, but SD cards do not.
Are you sure about this? I am virtually certain that modern SD cards, at least those from reputable companies, implement wear leveling. SanDisk has been doing it for more than decade, I think. Even if you assume no wear leveling, however, SD card sectors can do several thousand write/erase cycles before failure. That's equivalent to writing/erasing the card daily for years. I have no doubt that cards fail, and that they will all fail at some point, but I think the probability that card wear is the issue with so many people/cards experiencing problems is vanishingly small.
Wear leveling is not in the official SD spec as far as I know. If SanDisk has been using it, it would likely be on their top-line cards.
I think "several thousand" may be a bit generous - I've seen a variety of estimates as low as 10,000 cycles per sector. That said, I think card wear is probably not the primary culprit for all of the people having problems, though it may be for some.
If the buffer memory on the 70D is significantly smaller than that of the 680 or other multichannel recorders, then to me it's quite logical that the 70D is thus more "picky" with memory cards because cards because a device with a larger write buffer will be able to better compensate for the reduced physical performance of older and/or new but lower-performing cards that now have fewer good sectors to write to. That problem is amplified when recording multiple simultaneous data streams as is the case in audio recording. This is just a theory on my part, but I'd say it's a plausible one. Remember that a card's rating from the manufacturer is based on burst reads/writes; not continuous streams of data. Therefore a card may perform like a Class 10 in a DSLR, but absolutely suck wind in an audio recorder because of the difference in how it's being used - even if it's brand new. That's where media testing becomes important, and where I think Tascam should have had a good list of tested cards ready to go out of the gate upon product release.
I would say that if the buffer is too small to record audio (if that's the issue in the first place), that is a de facto design flaw in an audio recorder. I wonder if that type of problem would be fixable with firmware?
With respect to the bolded, these things are usually classed by sustained write speed. A lot of people use SD cards to record HD video, which is not only continuous, but also considerably more data than even four channel 24/96.
Interesting to see that a number of class 2 and 4 cards are on the European website. That would seem to imply that these issues (buffers and write speeds) are not the problem...
Yes, I would agree with you that too small a buffer would be a flaw. Back in my PC repair / building days I was obsessed with getting the very best CD burners. One of the things that separated the quality units from the cheap ones was the quality and capacity of their write buffers.
The idea that HD video is more of a data rate than high-res audio is a common misconception. The individual frames of HD video are pretty small megapixel-wise. Still high-res photography is probably more taxing, especially if you're shooting in burst mode, and according to Tom, audio recording is more stressful than both.
http://taperssection.com/index.php?topic=173960.msg2160546#msg2160546http://www.alphr.com/features/380167/does-your-camera-need-a-fast-sd-cardAnd some good FAQs from the offical SD Card people:
https://www.sdcard.org/consumers/speed/max_speed/index.html