Hi I've purchased a WD 1TB and I've formatted it into FAT so I can use it on both windows and mac. I have currently used up 40g from files from my PC. When I plug it into my macbook & selected WD 1TB as back up disc, here is what it says. "Are you sure you want to erase the backup disk WD 1TB? Erasing will destroy all information on the disk and cant be undone". "The disk must be erased before it can be used for Time Machine backups because it has an incompatible file system". What am I suppose to do now? Should I partition my external hard disk? If so how? Any help would be appreciated.