Huge mistake-almost 3Tb lost!!

Bummer. Yeah, as mgdm mentioned, you could try and use some data recovery software, but it doesn't look good. The idea with RAID is that the disks, although independent, work together to keep your data safe in the event of a disk failure. Without going to far into details, this is done through a combination of "striping" (writing your data across multiple disks) and/or "mirroring" (writing an exact copy of your data to all of the disks) and often leverages some type of error checking to reconstruct missing data ("parity") if required. So by necessity, the new disk needs to get in sync with the rest of the array in order to store the striped, mirrored and/or parity data, etc. Otherwise, it wouldn't be adding to the redundancy of the array. X-RAID hides the details of how to do this (formatting, etc). I think what you were expecting was more of a way to "merge" your two data sets together. Unfortunately, it doesn't work that way.