Background:
Manufacturer's SD card speed ratings are only true when you write one file to it, and let the card clean itself up with inactivity every few minutes.
This is always the case in a camera, whether in still or video mode.
For PCM audio in the DR-60D or DR-70D, you've got 2 or 4 files being written at once, i.e. the write position on the SD card is jumping around between
up to 4 different locations, and the writes never stop. SD card manufacturers rarely optimize for this case, which is why many cards end up on the
tested media list as "don't use".
The performance of the card is the problem. The DR-70D can't reduce the number of files written at once, that's it's reason for existing, the individual files.
A few messages here just say "Sandisk 32GB". Sandisk has many different series of SD card.
If a card worked once but is now not working, that's exactely the point - solid state media gets into a state where it needs constant erases and writes
instead of writes to already empty clusters. TASCAM tests media to this point to know if it's good enough, not just the fresh from the package condition.
Some SD cards, class 10 or not, drop to <1MB/sec when they have been well used. They're just not fast enough. The tested media list is there for a reason.
Watch out for counterfit cards as well - even a simple rebadge of a cheaper card with a sandisk label for stuff on eBay.