Same problem here, I think it's probably another bug. I already resolved the problem with mmc-opts (the problem with mounting without uid), checked both vfats (I have SD card) with fsck on other machine, tried various USB cables, it still doesn't work. I tried Windows 8.1 and Linux machines and it seems there is nothing relevant in the dmesg on both host / n900. After plugging the cable n900 LED flashes, then the dialog PC suite/mass storege is shown, I selected mass storage, then /dev/sdb and /dev/sdc is visible on the host. After while /dev/sdc1 shows, if mounted, it quickly disappears and the mount point is empty. But the mass storage correctly works from the bootmenu, so it's very probably not HW, kernel bug and there is probably some bug in the userspace (maybe related to hostmode switching?). Does anybody have any tip how to debug it?