Thanks for your answer Steve.

Sorry, /usr is on the same partition than / :

$ df -k
Sys. de fichiers               1K-blocs  Utilisé Disponible Uti% Monté sur
/dev/sda5                      96124904 50213412   41028540  56% /
udev                            1979492        4    1979488   1% /dev
tmpfs                            809836      972     808864   1% /run
none                               5120        0       5120   0% /run/lock
none                            2024584      216    2024368   1% /run/shm
/dev/mapper/data2vg-videolv   104031520 78035372   20753268  79% /video
/dev/mapper/datavg-optlv       15481840  7913832    6781576  54% /opt
/dev/mapper/data2vg-svglv      72822088  1403816   67748256   3% 
/Sauvegardes
/dev/mapper/datavg-baselv       5039616   430276    4353340   9% /bases
/dev/mapper/data2vg-photoslv   46814192 36375760    8079136  82% /photos
/dev/mapper/datavg-wwwlv       10321208  2057872    7739048  22% /www
/dev/mapper/datavg-qemulv       5039616  3778172    1005444  79% /qemu
/dev/mapper/data2vg-graveurlv  15481840  5293984    9401424  37% /graveur
/dev/mapper/datavg-vboxlv      27754364 15449564   10894960  59% /vbox
/dev/mapper/data2vg-userslv    15481840  8245632    6449808  57% /users


$ ls -al /
total 200
drwxr-xr-x  34 root root      4096 juil. 29 17:02 .
drwxr-xr-x  34 root root      4096 juil. 29 17:02 ..
....
drwxr-xr-x   5 root root      4096 oct.  14  2012 users
drwxr-xr-x  13 root root      4096 août   1 00:27 usr
drwxr-xr-x  13 root root      4096 août   4 22:38 var
....

$ ls -al /usr
total 332
drwxr-xr-x  13 root root   4096 août   1 00:27 .
drwxr-xr-x  34 root root   4096 juil. 29 17:02 ..
drwxr-xr-x   2 root root  94208 août   2 10:35 bin
drwxr-xr-x   2 root root   4096 mai   31 13:45 games
drwxr-xr-x  77 root root  20480 août   1 10:02 include
drwxr-xr-x 294 root root 114688 août   2 10:35 lib
drwxr-xr-x   6 root root  40960 mai   30 18:03 lib32
lrwxrwxrwx   1 root root      3 mai   30 11:44 lib64 -> lib
drwxr-xr-x   2 root root   4096 juin   2 15:11 libexec
drwxr-xr-x  10 root root   4096 juin  21  2008 local
drwxr-xr-x   2 root root  12288 août   2 10:35 sbin
drwxr-xr-x 480 root root  20480 août   1 20:50 share
drwxrwsr-x  14 root src    4096 juil. 29 17:02 src
drwxr-xr-x   4 root root   4096 sept.  1  2010 X11R6


Ok, I'm going to remove plymouth-x11 package which is installed


Le 04/08/2013 23:53, Steve Langasek a écrit :
> Is your /usr on a separate partition from / ?
>
> It appears that sometime between lucid and precise, the libdrm2 package
> regressed, and is now installing its library to /usr/lib/$arch instead
> of to /lib (or /lib/$arch).  This means that for any system with /usr on
> a separate partition, plymouth will fail to start up correctly at boot
> time.  This is probably the explanation for the problem you're seeing.
>
> The issues with the x11.so backend are ignorable.  You're not running an
> X server, so it's not as though this backend would be selected anyway -
> you should probably just remove the plymouth-x11 binary package from
> your system, which is not installed by default.
>
> ** Package changed: plymouth (Ubuntu) => libdrm (Ubuntu)
>
> ** Changed in: libdrm (Ubuntu)
>     Importance: Undecided => High
>
> ** Changed in: libdrm (Ubuntu)
>         Status: New => Triaged
>