I have a WD Passport external hard drive that I use to back up my computer running Windows XP. I have no problem reading and writing to the drive using Windows XP. I am also trying to back up my mother’s compter which is running Mac OS X v10.4.11 which is compatible with the hard drive. I have been able to back up the Mac on the hard drive in the past; however, the last few times I have attempted to use the hard drive on the Mac, it comes up as read only. I have installed the WD Macintosh + TURBO driver on the Mac but the hard drive remains read only. Why does the drive no longer allow the Mac to write to it?