I posted a previous thread about using my external WD HD as my backup which was in NTFS format. I reformatted the drive to accept the backup and it performed the backup. It took on the format of Mac OS Extended (Journaled). The problem is that I want to use this HD for both my PC and Mac. After reading forums discovered that using Disk Utility I should format to FAT32 which is also what I did. Now when I try to use it for the time machine back up it gives me this message "The disk must be erased before it can be used for Time Machine backups because it has an incompatible filesystem." I am using Mac OS X 10.6.3. My options in Disk Utility under erase are "Mac OS Extended (Journaled), Mac OS Extended,Mac OS Extended (Case-sensitive, journaled), Mac OS Extended (Case-sensitive) and MS-DOS (FAT).