Why can I not backup my Galaxy S2 with CWM ?

[AC Question]

Hi,

My Galaxy S2 (i9100) doesn't boot anymore (Samsung logo flashing definitely).
I absolutely need to retrieve data stored in the internal memory of the phone.
I'm using CWM but I get the message below. A 32 Go SD card is plugged in the phone with CWM software and 22 Go free.
I mount SD card from the CWM menu.

"ClockworkMod Recovery v5.8.1.5
E:Unable to open ums lunfile (No such file or directory)SD Card
space free: 22161MB
Backing up boot image...
Backing up recovery image...
Backing up system...
Backing up data...
No /sdcard/.android_secure found. Skipping backup of applications on external storage.
Backing up cache...
No sd-ext found. Skipping backup of sd-ext.
Generating md5 sum...
Backup complete!"

But no backup seems to be done.
Could you help me ?

Thanks a lot by advance !!