For some reason my PC (APPLE) cant see the phone although it did earlier !! Checked the micro sim in another phone and computer could see it. When I plug the Galaxy S into the computer I get the usb sign top left as normal then it asks me if I want to load files etc press MOUNT.....then it asks me to TURN OFF USB STORAGE !! Then before turning off USB storage make sure I have unmounted it...

I missing something.....

It happen to me sometimes after I have plugged in and out several times to the computer. Try unplug the USB cable, go Settings > SD Card and Storage > unmount and remount the internal SD Card. Now plug in the USB again see if it works, if it doesn't, try reboot phone.