Hello, I have a Western Digital 250 GB external harddrive, USB powered, that I which to use on my Windows Vista and my Mac OS X computer. I want to be able to read and write on both operating systems, but so far, I haven't been able to do so. I'm new to mac so i would like some help on this topic. Right now, i can only read and write on mac, because of the format the HDD is using right now. I want to be able to use it on both. Please help me... thanks!