I asked because I couldn't tell if MBR/GPT were RME-specific things or not. I looked into it reasonably well, and see that MBR is needed (although I think I have gotten GPT to work in the past). I'm on Mac OS, and it seems to be picky about what it will format as MBR-- my guess is has something to do with size, but I didn't find anything on that. I did find an older 250 GB EIDE (!) drive that formatted OK and worked in some tests. I still don't totally understand all the limitations and if RME has made them clear, but it works OK right now.
But my original issue is not resolved– the 3 TB drive can't be formatted with MBR (by Mac OS, anyway), even when I made a 100 GB first partition. The drive limit for FAT32 should be 8 TB. I did just realize it has a USB 3 port-- maybe UFX won't deal with that?