Have you tried to load that same card on a RockBoxed iPod, I got one of those 256gb cards and the file transfer failed even on RockBox which is interesting because the card is just storing files and not being loaded with any file transfer programs other than Windows Explorer.
It may work if we use disk images of albums instead of albums with individual tracks. (Like Dminor loads his music)
Before the patch, my initial tests on the 446g mSata V5.5 failed with RB and I haven't been able to get to test the patch yet. I really want to know if there is a difference in the song limits for the different media, and so I have been testing the SD, mSata, and HD during the last week. After I finish these tests, I will test the 446g mSata with RBpatch.