Some questions and suggestions:
-Can you view the sdcard contents within android or through CWMs "install zip" function?
-Make sure you backup everything on there (within android via wifi somehow, through adb pull, etc)
-Once backed up, attempt to flash Slim ICS (as it has a working MTP and UMS functions), go into settings -> storage -> menu button -> advanced -> toggle between MTP and UMS and see if it works
-If this works, switch it back to UMS and go back to another stock ROM
-If it still doesn't work, format the sdcard through CWM and retry
-Still no success, post back and i'll have more ideas ready (hopefully)
Device: Nexus S i9023