HELP: My Book Mirror Edition 1 TB

not sure what going on but this drive is un-plugged from Mac and  the blue light on drive is showing activity? 

also had to run “Verify disk” got erros  “Invalid Sibling link” and “Invalid Key Length” and many others then this drive just diapeared from “Disk Utility”…  ran it again then said Disk Utility couldn’t repair disk… so I take it as this is a sign of failer, but "Western Digital Drive manager says, drive is healthy …

WD Drive Manager doen’t work !!

I don’t know much about Macs but I think this is a RAID drive with 2 HDs. RAID 1 is mirrored and RAID 0 adds the capacity of the 2 drives and treats it as one.  I don’t know where to start but with RAID 1 if one drive of bad the other should be recoverable. Sorry I can’t add much more at least it gives you a starting point. I found this if you don’t have a user’s manual