I don't know what causes this, but when I first flash a ROM I can see the internal storage as well as the external storage when I connect my phone via USB to my computer. I don't know what I did but now it doesn't show up anymore. Some people have said that simply unchecking "USB Debugging" would fix this, but that does nothing. Sometimes when I connect the phone the MotoHelper installer (which is only about 8mb in size) shows up and asks if I want to install the MotoHelper software. Does that have something to do with it?