Trying to recover data from an external RAID drive
If your RAID is 4 TB from 2 x 2 TB then it is RAID 0, a stripe set where one 2 TB drive is an "extension" of the other. Each 2 TB drive won't work by itself. If any of them fails, the other is useless. This setup is useful for write and read speed, but it decreases confidence in the set.

So, if you pull any of the 2 TB drives from the set and tries to use it, it won't work. You need to repartition the drive as a single drive (no RAID), but then the other drive will be useless forever (you destroyed on of the drives in the set.

If it was RAID 1 (mirror), you would have 2 TB from the 2 x 2 TB set, but each drive would be an exact copy of the other and you should be able to use each one by itself. This setup seems a waste of space but it increases confidence in the set since any 1 drive failing would still allow you to access your data.