Sorry for necroing such an old thread, but i gotta ask, what format do you use on that 128Gb card?
i'm coming from cm12.1 by one of the developers on xda, and over there they have support for exfat and ext4 support for sdcard (although the latter is kinda buggy). I know for a fact that global rom from en.miui.com doesn't support exfat (need another tool with root to mount it). i was thinking about installing the build from this forum, the latest being 6.5.19 and i was wondering if the rom from xiaomi.eu does support exfat for external sdcard.