with my home Linux PC I made a fat32 partition on my sd card for music 23gb. Then 100gb ext4 for possibly /home for Linux (if i can work it out) when i finally flash.
The music app (i installed vlc) found it, scanned the directory and albums np.
I would have preferred to image a partition on the sd card than flashing the phone (just to try OS out, bit like using a liveCD). But this really is new territory, it'll probably be an option in a years time .