Problem.

A Client had an network attached WD MyCloud 3TB Drive and had attempted to attach an external 3TB disk (re-purposed with an external JMicron Hard drive caddy)

Regardless of formatting it as NTFS, exFAT  -the JMicron controller was being recognised but the 3TB Disk partition swasnt

Solution.

Attach the disk to windows and ensure an EFI partition has not been created.

If an EFI partition exists, remove it using diskpart from an elevated command prompt.

(use command: delete partition override as the EFI partition will be marked as protected)

This is what worked for us in this instance and hope it helps others.