^ I would still like to know why recording four channels to an SD card is so difficult. There are quite a number of recorders that can do it, some of which can record many more than four channels, and some of which are Tascam products. Just seems like what the Dutch call a "smoesje", which is some sort of lame excuse...
I don't know if this is the answer or not, but I'm wondering if the issue might not be limited to only the write speed on the card but also be related to how the unit is designed to handle data transfer.
I don't remember for sure which unit it was, but I once had a recorder that indicated on it's display when the speed of data being created by the recorder exceeded the ability of the device to write. When that happened (somewhat regularly), some kind of a buffer memory kicked in for the data to be stored temporarily while the write process caught up. I figure the card bogged down as it was searching for clean sectors to write onto. I also remembered that, there would be very rare occasions where I'd get a skip in the music. There were no drop-outs or digi-glitches, but there might just be a second or two where it was obvious that the recording process stopped and even though the recorded music was continuous (without drops, pops, or noise) it would instantaneously skip. In those cases, the buffer filled up and, once full, there's just no place to put the data so a second or two of music was lost forever. This was rare though.
Now, I'm not sure how your 'typical' digital recorders designed, but it's becoming apparent from Tomuo's responses and the limited number of acceptable cards, that this DR70D design is far more sensitive to accurate data transfer (perhaps due to data backups or bottlenecks?!?) than other recorders. Maybe the DR70D doesn't even have the capability to buffer data, whereas the others do?!? Since the cards on the list are either very fast or sophisticated cards (the 'extreme pro' cards for example), from Tomuos response above it seems that there's only a handful of cards that can stay ahead of the recorder...or perhaps these card design itself has the buffering capabilities built into the card.
Anyway, thinking this through some, if a typical recorder can buffer data and the DR70D doesn't, that could explain why the same cards work in other units when they don't work in this one.