For both my field recording and my testing, I had the MP-6 set to advanced mode, recording at 24/48. Field recording used 4 mic inputs and a LR mix track (channels 1&3 panned hard left, 2&4 panned hard right), in testing at home I added channels 5-6 as well. Line output using the LR mix track, using a mix of channels 1&3 for left and 2&4 for right (in the home testing with channels 5-6 added, I didn't send those to the LR mix track as it was different source material). LR mix signal routed post fade (blue).
In all cases, the dropouts occurred at the same time across all 4 (or 6) channels as well as the LR mix track. I had external recorders attached to the line output and the headphone output (headphone on home testing only) and dropouts occurred at the same points on the external recorders as on the internal MP-6 recording.
No idea where and when I got the Sony card, or even where I was using it. I gathered up all the high capacity SD cards I had from various recorders and cameras to use in the MP-6 and lost track of what was used where. I don't recall having any trouble with the Sony card, or any other.
Wondering if there was some glitch on the card, earlier today I did a low level erase and re-formatting on it. First time through, it failed for some reason, so I did it again which seemed to work without incident. Didn't think to try disk first aid before doing all this, but tried disk first aid after the second low-level reformat and it didn't show a problem. Then I re-formatted it yet again in the MP-6 and set things up to re-test. After about an hour of testing, I didn't have any dropouts on the recording to the Sony card, and no dropouts on the two external recorders patched into the headphone and line outputs (unsurprisingly). Only 4 channels plus the LR mix track for this test, I didn't set up channels 5-6 this time around.
Don't know that I will ever trust or use the card, but the failures seem to have gone away. I've ordered a Sandisk extreme card for use in the MP-6. I'll test that card when I get it, which I assume will be fine. With any luck I won't see any more card failures, though I don't know that I'll bother running the Sony M10 as a backup recorder since I don't see much point.