Can someone help me with understanding why Time Machine is starting over on backing a drive? Here's my set up - 500GB SSD - main OS with about 450GB of data 2TB HDD - secondary drive for bulk storage with 998GB of data. They are both installed internally into the computer. What happened I think is that one day I removed the 2TB HDD to access it's data from another Mac, leaving the original Mac without that drive when it did a backup. Now that I have put that drive back in, it is proceeding to back up 998GB of data as if that drive is completely new, even though nothing has changed on the disk. Does anyone know why this is happening? It's going to completely fill my back up drive and that's a whole lot of reading and writing of data for no good reason. Is there a way to prevent this next time, besides not turning on the main machine while the disk is missing?