I am a consumer in this case, you are going to see continued piling on of negativity from my end until this problem is resolved to my satisfaction.
My first bad write was on this card:
ADATA 16 GB SDHC Class 6 Flash Memory Card 16GSDHC6 (Blue)
purchased in August of 2009 from Amazon.com
The card has been heavily used, and may have been formatted in my Panasonic camera before use in the Tascam. The error I found was a failure to write glitch-free files in 4-channel operation at 24/48.
My second bad write was on this card:
SanDisk Ultra SDHC I CL10 30MB/Sec with a black, white, red and metallic silver label
That was attempting to have the DR-70D write a single stereo file at 24/48
There should be a new tested list due out (last one was Februrary, mostly done every 6 months), I agree that 11 cards tested is too few.
The DR-680 has been out a long time, the larger channel count means people only buy the larger cards that have been out that long, and users have probably been sticking with known good cards.
Again, let us know what cards you have that you expect to work that don't. The choice is much larger now than it was when the DR-680 came out.
Also, what is the comon problem, I couldn't find a single report here which told me exactely what card they used and what problem they ran into? I see a lot of posts piling on the negativity, but nothing I can get real info from.