Is the 300 GB hard drive SATA or PATA? If you using the older PATA drive it might be a driver / registry problem. I've had this problem and a friend also had the same problem with PATA hard drives over 137GB (48 bit LBA). The issue is that some drivers require a registry setting set to enable 48 bit LBA on the controller. It gets worst, the drive will look like it is working just fine when you format it without this registry setting but it won't start having problems until the drive starts using the space above 137GB. We also found that after a Windows Updates the registry setting could be gone.