Comment 10 for bug 953875

Appears to be random - sometimes it mounts, sometimes it doesn't. I think it will be necessary to use UUID's?

$ uname -a
Linux .. 3.8.0-32-generic #47~precise1-Ubuntu SMP Wed Oct 2 16:19:35 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux

$ cat /etc/crypttab
cryptswap1 /dev/sda7 /dev/urandom swap,cipher=aes-cbc-essiv:sha256

$ sudo cryptdisks_start cryptswap1
 * Starting crypto disk... * cryptswap1 (skipped, device /dev/sda7 does not exist)... [fail]

I think part of the issue is that /etc/fstab uses UUID and /dev/sdX# can change on boot (e.g. from sda# to sdb#).

My fstab (UUID's modified slightly):
# <file system> <mount point> <type> <options> <dump> <pass>
proc /proc proc nodev,noexec,nosuid 0 0
# / was on /dev/sda5 during installation
UUID=8ed19201-fabf-4364-a507-XYZ / ext4 errors=remount-ro 0 1
# swap was on /dev/sda6 during installation
# /dev/sda7/ none swap sw 0 0
/dev/mapper/cryptswap1 none swap sw 0 0

$ sudo blkid
/dev/sdb1: LABEL="SYSTEM" UUID="C2E04BEXYZ" TYPE="ntfs"
/dev/sdb2: LABEL="Win7" UUID="32B682AXYZ" TYPE="ntfs"
/dev/sdb4: LABEL="RECOVERY" UUID="120C19F4XYZ" TYPE="ntfs"
/dev/sdb5: LABEL=".." UUID="8ed19201-fabf-4364-a507-XYZ" TYPE="ext4" <===
/dev/sdb6: LABEL="..." UUID="6C94DF5A9XYZ" TYPE="ntfs" x

Note that all of those are are mounted as /dev/sdb#. If I boot with and external USB drive, that drive is mounted as /dev/sda#:

/dev/sda1: LABEL="GGNTSF" UUID="4868D7FF68XYZ" TYPE="ntfs"
/dev/sda5: UUID="27bd7267-ab0b-48f4-8a56-XYZ" TYPE="crypto_LUKS"
/dev/mapper/udisks-luks-uuid-27bd7267-ab0b-48f4-8a56-XYZ-uid1000: LABEL="ext4" UUID="86d20931-6b90-4082-9b14-XYZ" TYPE="ext4"

Now if I modify crypttab:
$ cat /etc/crypttab
# cryptswap1 /dev/sda7 /dev/urandom swap,cipher=aes-cbc-essiv:sha256
cryptswap1 UUID= /dev/urandom swap,cipher=aes-cbc-essiv:sha256

and

$ sudo mount -a
$ sudo cryptdisks_start cryptswap1
 * Starting crypto disk... * cryptswap1 (starting)..
Attaching loopback device failed (loop device with autoclear flag is required).
device-mapper: rename ioctl failed: No such device or address
Command failed
 * cryptswap1 (started)... [ OK ]

$ sudo dmsetup status
udisks-luks-uuid-27bd7267-ab0b-48f4-8a56-67f8713b56ea-XYZ crypt

It is attempting to use the UUID of /dev/sda5, the external USB drive partition rather than /dev/sdb5: LABEL=".." UUID="8ed19201-fabf-4364-a507-XYZ" TYPE="ext4" - the partition that I booted into.

I'll reboot without the external USB drive connected.