|Look at Windows Disk Management and see if the capacity for that drive is listed as 2TB there. |
Depending on what type of RAID array that would have been configured that number could be something different.
For instance, if your primary hard drive and the eSATA were configured as RAID-0 the array capacity would equal the sum of both drives. That is one possibility.
I have not used an eSATA connected drive so I am unsure how it should appear. Is the drive functioning as expected?