Does that mean that I need a 6tb disk? Right now, I have a 1tb hard drive in my iMac. I am looking at many different options of expanding storage but one of the cheapest options is to simply add a 2 or 3 tb hard drive to the iMac via usb. I am currently using a 2tb hard drive for time machine backups but everything I read suggests that my time machine drive will not work if it is not double the size of the original drive(s). So does this mean that if my iMac + usb drive = 3tb, I would need a time machine disk of 6tb? If so, is buying a NAS the only option for this solution since there are no 6tb drives?