SC Caos Theory Problem with USB
 

hi,
i unplug the dvd drive , and i connect my usb dvd-rom with SCPT.
I have launch the game once and I have played, the second time more is then not left again giving error in the "copy protection".
I have re-installed the game, before turns the game has gone, then the successive times not more!
Why?
How I can resolve my problem? Thanks