Hello, I have been using an external hard drive as my time machine for years, then suddenly the back ups started to fail. So I went in to Time Machine settings and found that the reported size of the next back up was greater than space that existed on the drive, then I noticed something odd, my Imac has an internal HD of 250GB of which 94GB is used. However the next back up had a reported size of 490GB! All my external drives (one for iTunes, one for video content) are excluded, and yet the estimated size of the back up is 490GB from a 250GB drive... So, I erased the drive, so now it's empty and ready to take a back up, but with such a huge estimated size it will not fit... Any ideas why suddenly Time Machine thinks there is 490GB to back up from a 250 GB system drive with only 94GB used?