Currently Being ModeratedAug 1, 2012 2:31 PM (in response to vmpillet)
You can clone your OS to an external HD, boot from the external and use DU from there to repair the internal drive.
If by USB you mean a thumb drive, it's possible, but you need to install SL on the drive and then boot from it. Which would require a working DVD drive anyway!
If you can install on a thumb drive from another Mac (using the retail disc), then update it to 10.6.8 that may do the job. Otherwise go with the external HD.