Comment 9 for bug 481626

I am also experiencing this issue.

Per the tail of this forum thread: http://ubuntuforums.org/showthread.php?p=8110630
I was able to stop the continuous disk access by restarting the udev service.

Here's "top" at first login after a reboot:

  PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
 3305 root 20 0 5988 3744 2240 S 8 0.2 1:06.80 devkit-disks-da
  880 messageb 20 0 3280 1608 748 S 8 0.1 0:56.49 dbus-daemon
 3320 schaefer 20 0 6496 3040 2380 S 5 0.1 0:39.04 gvfs-gdu-volume
    1 root 20 0 2640 1556 1120 S 4 0.1 0:31.39 init
  504 root 18 -2 2372 848 312 S 4 0.0 0:32.33 udevd
 3287 schaefer 20 0 27780 11m 8676 S 4 0.6 0:26.37 update-notifier
 3283 schaefer 20 0 17344 6576 5152 S 3 0.3 0:20.43 gdu-notificatio
  483 root 20 0 2584 1220 944 S 2 0.1 0:11.55 mountall
  498 root 20 0 2148 748 568 S 1 0.0 0:09.24 upstart-udev-br
  500 root 16 -4 2704 1144 300 S 1 0.1 0:05.92 udevd
 1126 root 20 0 92532 20m 9368 S 1 1.0 0:08.95 Xorg

Here it is after "sudo restart udev":

  PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
 1126 root 20 0 108m 23m 9.9m S 3 1.2 0:22.10 Xorg
 7388 schaefer 20 0 241m 81m 24m S 2 4.1 0:10.36 firefox
 7386 schaefer 20 0 2468 1180 884 R 1 0.1 0:00.42 top
 3215 schaefer 20 0 83560 26m 7804 S 0 1.3 0:04.30 compiz.real
 3217 schaefer 20 0 60676 19m 13m S 0 1.0 0:04.29 gnome-panel
    1 root 20 0 2640 1556 1120 S 0 0.1 0:42.54 init
    2 root 15 -5 0 0 0 S 0 0.0 0:00.00 kthreadd
    3 root RT -5 0 0 0 S 0 0.0 0:00.08 migration/0
    4 root 15 -5 0 0 0 S 0 0.0 0:00.01 ksoftirqd/0
    5 root RT -5 0 0 0 S 0 0.0 0:00.00 watchdog/0
    6 root RT -5 0 0 0 S 0 0.0 0:00.01 migration/1

A check with "ps" indicates that devkit-disks-da did NOT exit when udev was restarted, but did cease consuming large amounts of CPU and spinning the disk. There may be intersecting bugs here.