We have seen this behavior when the motherboard does not have UEFI. Would you please check on BIOS is the motherboard comes with UEFI?
UEFI is needed when your hard drive or your partition is larger than 2 TB.
If you find this feature on BIOS, you would have to enable it, but it will be necessary to reinstall the operating system and recreated the RAID.
In your post, you mention that you are not able to boot to the operating system? Was the OS installed on that RAID?
Thanks for your response
I boot off an SSD. No, I am only just trying to create a RAID mirror for data storage not to boot off.
No, after the failed attempt (small partition creation) the OS just told me that the driver load failed? But this appears to have resolved itself on a later switch on.
After the RAID mirror creation fail I deleted the RAID volume and tried to use the 4Tb drives as NON-RAID JBODs and got the same size discrepancies? When prompted by the Disk Management program (MBR/GPT?) I selected GPT.
I must have UEFI (or partial EUFI if there is such a thing) as I have 2 other internal 3Tb drives AND an external 4Tb drive ALL working perfectly.
I contacted the manufacturer (Gigabyte) who confirmed I would have trouble with discs > 2.2Tb (which I am NOT) so they seemed as confused as me
I'm completely puzzled.
Anything else come to mind?