Comment 25 for bug 1489855

look into the code, u can also see the fault posted debug log above:

when using a USB HDD(/dev/sdb) partition /cdrom(/dev/sdb1 vfat) casper-rw(/dev/sdb2 ext2)

1. mount /dev/sdb1 /cdrom (/casper is exist(will load filesystem.squashfs lately))
2. find casper-rw(file and partition in same time(cause bug))
   casper.setup_unionfs-> casper-helper.find_cow_device
   for each /dev/sdb*
      a. find file
          a.1 /dev/sdb* not mounted(mount /dev/sdb* to /casper-rw-backing)
          a.2 /dev/sdb1 mounted(remount -rw on /cdrom)
          then check the casper-rw file's existence
          ***after check(not exist), it will umount the /dev/sdb*, for /cdrom will be umounted, here is bug ***
      b. find partition just check LABEL = casper-rw
    in order of /dev/sdb1 /dev/sdb2, so /cdrom be umounted

workaround may be:
  1. put casper-rw(/dev/sdb1) before /cdrom(/dev/sdb2) partition
  2. or modify find_cow_device, 1st find partiton, 2nd find file(don't do in the same time)