Thank you for having replied to me Rockalinux.
I tried to follow your advice but another problem comes out now :
There is no line in the '/var/log/messages' with 'USB mass storage device'
The only lines dealing with 'USB' say something like that :
May 31 16:10:56 thierry-desktop kernel: [12766.757238] usb 1-10: configuration #1 chosen from 1 choice
May 31 16:18:59 thierry-desktop kernel: [13288.776099] usb 1-10: USB disconnect, address 4
These are the last two lines of the file.

I had a look at '/dev' and there are listed (among other these entries :
scd0
sda
sda1
sda2
sda5
(further on...)
usbdev1.1_ep00
usbdev1.1_ep81
usbdev2.1_ep00
usbdev1.1_ep81

i don't know what to do, which entry to choose, if any...

Thank you for your help.