Thanks for the links jeyare.
I am still baffle by the fact that after seeing high IO on my NAS and restarting it, disks 3 and 4 were no longer recognised by the NAS enclosure or anything else.
I have already tried smartctlools and mdadm but the disks cannot be read so I cannot do much.
I cannot check or mount the disks as they are not recognised when connected.
The serial numbers or their storage capabilities are not displayed by smartctlools, so trying to rebuild the RAID,even with just 3 drives does not seem an option.
When running the Data LifeGuard Diagnostic utility from WD the disks serial number or capacity is not shown and any test run on them comes back without errors in a matter of 1 or 2 seconds so its not reading them.
I even tried cloning the drives with the WD offered Acronis software but this failed as the software does not see the disks as WD ones. I can use it with the other 2 disks.
I did also try cloning the drive with Clonezilla but I had the same issue.
I don’t know what else to do