Maybe there are more restrictions than that. For instance, if any drive exceeds the original 2TB cap, then you are limited to a maximum of two drives. If every drive stays under 2TB, then you can have three drives. (In that case, the cap for three drives working would actually be 6TB.) I don't know why they would restrict the usage that way. It is just a theory, based on the results people have posted here so far.I'm not sure why, but I couldn't get the 1TB + 2TB + 4TB drives to work together. It should have worked with the total storage of 7TB.