OK, I just did another test on my suspect SanDisk card. Didn't quite fill the card this time, but recorded just under 10GB. This time I only did a Quick format, power cycled multiple times, and made tracks of varying lengths. Started with 4 tracks 24/96, then switched it up to 2 track 24/48 with DUAL REC which is how it was set when it started glitching on me the other day. Just went through all the files in RX, and all is well with no errors.
Here's something that may or may not be significant: after formatting this card in my computer with the SD Formatter tool, all subsequent test recordings I've made in the 70D have been after it was upgraded to the new 1.12 firmware. In my rush of editing the band show and doing multiple things at once while being frustrated that my other tracks were trash, I just went ahead and did the firmware update before considering doing any of this testing.
The firmware update does say "operational stability has been improved." That really could mean anything. I'm curious to see if Tonedeaf gets any problems with this card on the older firmware. I don't really want to downgrade mine - I never really trust firmware downgrades, maybe irrationally so.
Here's a thought: Several of you have said there may be a problem with the firmware in this unit and that's why it has problems with certain SD cards, even ones that are fine in other 4-channel recorders. How about doing some tests like I have done with those cards on both the 1.11 and 1.12 firmware, with a fresh Erase Format each time? The card may still end up being "bad", but it could be interesting to see if somehow after the new firmware, now it isn't.