Dyconn Quartz 4 Bay HD Enclosure Cannot be Repaired?

I have a Dyconn Quartz 4 Bay HD Enclosure-WD Black External HDs. For the second time, with separate WD Black 2 TB drives in a Dyconn Quartz 4 Bay enclosure, I have gotten a message that the disk cannot be repaired. These disks were used as backup disks for a particular external HD that seems to be working fine. So far, no problems with other WD Black drives in the same enclosure, either as source or backup drives. OS is Apple Yosemite. Backup software is Carbon Copy Cloner. Don't recall a problem like this before updating to Yosemite, but didn't have this problem until after I had been using Yosemite for a while. Yosemite and CCC are latest versions. Question is, how to proceed with troubleshooting, given that the problem has shown up with two HDs that have not been used extensively or experienced any abuse?

BTW, I can access the data on the affected disks and CCC gives me an error message that certain files were not backed up.