Internally the device contains two matching 1TB (1000GB) disk drives which are configured to duplicate (mirror) each other (called RAID 1) to give a total usable storage of 1TB.
In the event that one drive fails, all the data is still on the other drive, and as soon as the the failed drive is replaced the device will automatically duplicate all the data (re-mirror) from the good drive to the new drive. The replacement drives must be the same capacity as the remaining drive, and prefereably the same manufacturer.
NCIX is a good place to start to find a replacement drive.
The installed disk drives are: