Kind of a strange question. The answer is how ever much you want. When this feature was first released they said they were not supporting HDDs larger than 750GB, but many have added 1TB drives with no problem.
I have 2 500GB My Book essentials. About 3/4 total filled between the two. I find the Western Digitals to work great. I thought about the 1TB drive, but it was cheaper to get the 500GB drives.
Kind of a strange question. The answer is how ever much you want. When this feature was first released they said they were not supporting HDDs larger than 750GB, but many have added 1TB drives with no problem.