When an external hard drive is initialized or formatted on a computer system running Windows 7 or Vista, you may have either converted or created a GUID partition that will not be readable by Windows XP. If it has a GUID partition (GPT Protective Partition), it will look similar to the image (Disk 1) below when you check Disk Management. If you don't see the drive in Disk Management, something may be wrong with the drive and you should Contact Us for additional assistance.