So i went with formatting the 128gb microsd card as UDF and its working great on the Xperia X (with caveats). The Xperia recognized all of it, and i can plug the phone in to windows 10 MTP and copy my FLAC files. I use QuasarMX to play them, as it allows you to choose the directory to scan for. Camera can also save photos to SD as well.

The caveats are that the stock Sailfish Media and Gallery apps aren't seeing the card, but i wonder if its just because i need to tweak which dirs the tracker scans. Haven't figured that out yet.

Thanks again DrYak for the idea about UDF. There seems to be little discusion about using UDF and Sailfish, but so far its working well for me.

For those interested, i ran a simple format command with admin rights on Windows 10 cmd prompt (where my microsd was drive F: ) :

format F: /fs:UDF

https://technet.microsoft.com/en-us/...(v=ws.11).aspx