Maximum Harddisk Capacity

There are only disks up to 3TB in capacity on the compatibility list at this time: http://kb.netgear.com/app/answers/detail/a_id/20641

However some users have found 4TB disks work fine for them.

In the future even higher capacity drives will be released. Currently there are no known reasons why higher capacity drives still would be incompatible but it's possible they my not be compatible.

The ReadyNAS Ultra 2 uses the EXT4 filesystem which theoretically supports volume sizes much higher than you will ever have in an Ultra 2 unit.

With two 4TB disks using RAID-1 you would have a volume of about 3.6TB which could theoretically be expanded to about 11.6TB without doing a factory reset.

Do note that a new model the ReadyNAS RN312 has been released. This has a newer faster CPU, double the RAM, runs a newer OS and has 4TB disks on its compatibility list. You may wish to consider buying one of these if it fits within your budget.

Thanks for the helpful reply.
The only reason I want to buy an Ultra 2 is that there is a nice discount in my favorite online shop.
I will pay 355 Euro (465 $) for an Ultra 2 and 2x2TB (seagate) hard drives.
The reason I asked the question was that I wondered if it would be possible for me to expand the capacity in the future for possible needs.

Thank you so much. That reply really helped a lot.