Tx Brian for the info 
but the service procedure didn't help me 
on the other hand you were getting close with the usb plug/unplug it is indeed related 
  
in the mean time I got an answer from Lacie (see below), and it does the trick for me 
  
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For Big Disk Triple Extreme or Big Disk USB or Bigger Triple Extreme: 
(using TI USB bridge). 
  
Please be sure to plug first the power supply and later the USB. 
If you don't the drive may not be sometime recognized by the OS, asking 
for a driver (TUSB 6250). 
In this case, please unplug both USB and Power cable and then plug the 
power cable first and then the USB cable. (Unplug only the USB cable 
doesn't reset the board). 
  
  
Please include all previous messages if you reply. This information will 
be required by the email support team to further assist you. Thank you. 
  
Answers to most common questions can be found in the manual on the CD 
that came with your product or in our FAQs: 
http://www.lacie.com/support/faq/ 
  
  
Regards, 
Technical Support 
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en francais cela donne on debranche alimentation et usb du disk externe 
puis on rebranche l'alimentation, enfin et en dernier lieu seulement  on rebranche USB 
  
voila,a+