Somebody help iMac 27 does not see My Book Studio Station 2TB. There was it after replacement of disk A and inserts of a new disk on 2TB were then returned into place by an old disk and iMac has ceased to see old HD disks. Why. I from Russia. Why?

Make sure that the disk you are using as a replacement is the same model and capacity than the original, otherwise it will not be detected by the My Book Studio.  You may need to use WD Drive Manager in order to reconfigure the RAID:  How to install WD Drive Manager and set the RAID mode on a Mac