I have a Bolt 1 TB unit. I want to add an external storage unit in the range of 2 to 4 TB. I note that Tivo site stresses "a Tivo certified" unit. However, these do not seem to be available and I have noticed from various threads people are using various hard drives. If I want to connect a 2TB external hard drive, what would be recommended? What are the desired specs? Does the Bolt automatically reformat the drive or should it be previously formatted and if so what format?