I'm only seeing this with one of my sd cards.

After a factory reset, with an SD card in that contains a log dir, fab4 crashes.

The next reboot works fine (Felix suggested "I think that has to do with udev gathering information only the first time after a reboot")

I see the error:
FAT: Filesystem panic (dev mmcblk0p1)
   clusters badly computed (124 != 46)
   File system has been set read-only 8:45


The same card sometimes gives the following when I insert it:
    fat_get_cluster: invalid cluster chain (i_pos 3677299)
FAT: Filesystem panic (dev mmcblk0p1)
    fat_get_cluster: invalid cluster chain (i_pos 3677299)
Jan 21 08:58:14 kernel: FAT: unable to read inode block for updating (i_pos 3677299)
Jan 21 08:58:14 kernel: FAT: FAT read failed (blocknr 29)
(over and over)

Removing the log dir fixes the problem (apparently since messages is no longer then being written to the sd card)