Trying to recover data from an external RAID drive I have a 1 TB Western Digital external hard drive that is old but works fine, I also have a 4 TB WD RAID (2 x 2 TB hard drives in one casing) that stopped working which I'm trying to recover. I must say it turns on when I plug it, but mac doesn't recognize it.

I pulled out HDD #1 from the 4 TB RAID, then connected it via the SATA connection to the housing of the 1 TB. Physically, it connected without a problem, but my Mac doesnt recognize the "new" drive.

Any ideas what I should do in order to see the data from the 2 TB HDD?
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.