It does sound like your internal storage has been corrupted, but I don't know if I would try to format it via your PC. this is not actually an "SDCard", it is the internal storage on the device (which is a SGH-T959, not an I9000).
I would revert back to STOCK ROM using ODIN and return the phone under warranty. If you are already having issue with the internal storage, I would want a new device.
Now I guess you could try and format it via your pc (I dont know if you can even access it when its mounted via USB because I never mount it) but I, personally, would not, I would get a new device.
For reference, the i9000 is the international version of the Galaxy S phones. None of the US carriers sell the i9000. They sell different versions of the the Galaxy S phones. For example, the T-Mobile Vibrant is the SGH-T959, The Verzion Fascinate is SCH-i500, the AT&T Captivate is SGH-I897. While they are ALL Galaxy S phones, they are all also different devices. They have different hardware, like some have a front facing camera, led flash, 4 buttons, etc.