I did a show last week where this was a problem. It was not my board (I do have some smaller Presonus mixers). I thought the 32GB card would be enough because we only had to track 25 channels.
When we got ready to test the recording, the board displayed a very small run time -- about 40 minutes as I recall. I think there was a prior recording on this SD card (we actually had 2 SD cards and I think they both had prior recordings on them.) Unfortunately there is no way to format directly from the mixer. There is not even any way to delete the unneeded sessions. That's a HUUUUUGE problem.
I happened to have a Surface computer I could use to delete the files off one of the cards (which was a MicroSD). But when I inserted in the board after deleting, it still showed about 40 minutes. Maybe the card kept the files in a recycle bin. I don't know. We ran out of time, so we couldn't do a multi-track recording. That's a real bummer. You really, really should allow the user to delete old sessions directly from the mixer.
But even with a freshly formatted card, this would not have worked. Our show ran 2 hours, 5 minutes without an intermission. I know that's extreme, but that's the real world.
I understand the licensing issue. Any chance you could support FLAC, as Soundcraft does? FLAC would make 32GB practical for most people.
The real kicker is that I have a laptop dedicated to USB tracking and it was already set up with the Presonus drivers. If I would have known this SD thing had so many problems, I would have brought the laptop and been done with it. That laptop is all SSD and I can record 32 tracks 10 hours or more.
I also note that Zoom is able to include SDXC support on most of its recorders. I wonder if there are any other licensing arrangements that would make this affordable.