Hi Bumpydog99.

what about Device Manager? do they show up there? if not scan for changed Configuration (Device Manager > Right Click).

Meant to say, these are Mediamax (WD) drives and the BIOS identifies them as WL5000GSA12872

I had already tried that and, no, they do not show up there either. All I see is my 1TB SSD and ST2000DX001 2TB HDD (which are SATA ports 3 and 4 respectively - SATA 1 and 2 are optical drives). All of these devices show up in the BIOS at startup (plus, of course, my two 'missing' drives in SATA 5/6)

what SSD Model is it exactly?

are SATA 5 and 6 on the Motherbord also the SATA Express Ports?

do you have Hard Disk Menu in the BIOS Settings where you can Enable / Disable Drives? maybe you have to activate them first.

My SSD is a Sandisk Ultra 960GB. I'm running BIOS A11 and I can see no way of individually enabling or disabling SATA ports. I have no idea whether SATA 5/6 are Express ports or not. I might try swapping ports 1/2 with 5/6 to see what (if anything) happens. One thing I have noticed, which could have some bearing, is that, after a cold boot, the machine seems to hang at the point where the blue window is on the screen and the 'circle of rotating dots' stay like that indefinitely. If I force a power down and restart, it boots up fine (although it can't see the additional SATA drives).

OK, just checked further into the BIOS and the boot mode is set to Legacy!! I plan to now backup the SSD and reconfigure it as UEFI and restore it and see where we go from there.

Thanks to both of you for taking the trouble to reply.

*3 - 6 aren't SATA Express ports

I might check that the Bios is set to UEFI. I don't have experience with drives larger than 2Tb, but I have formatted drives to GPT using DiskPart in a Command Prompt. I wouldn't try this until you're sure it is acceptable or appropriate to do; note that List Disk can tell you what format the drive is currently in ...

"Master boot record (MBR) disks use the standard BIOS partition table. GUID partition table (GPT) disks use unified extensible firmware interface (UEFI). One advantage of GPT disks is that you can have more than four partitions on each disk. GPT is also required for disks larger than 2 terabytes. You can change a disk from MBR to GPT partition style as long as the disk does not contain any partitions or volumes.

See also:

1. Convert MBR to GPT using Diskpart

This method you have to backup all your data and delete all partitions and volumes. Then use the DISKPART command.

• Open command prompt and type in DISKPART and press Enter
• Then type in list disk  (Note down the number of the disk that you want to convert to GPT)
• Then type in select disk number of disk
• Finally, type in convert gpt

It is true that the middle and bottom SATA data ports 3 4 5 6 are the boot ports (top two for DVDs etc). I might switch my new drives into ports 3 and 4, switch the working drives into 5 and 6 and recheck. Or, remove all drives accept the working drive with your OS, then insert a 5Tb drive, example > OS drive into port 3, new 5Tb into 4 then 5 then 6 then 3 if that's what it takes to test

* for laughs & giggles, try them in ports 1 &  2 by disconnecting the data cables and moving them up a row, see what happens

The closest result I got for a large drive was this post, which had advice but was unresolved, therefore this post and the one linked may be of no help today, sorry if that is the case or you already knew all of this info > Area 51 Factory Reset Windows 7, Upgrade to 10 - Cannot Find All 6TB Drive Space

okay. try to swap the Ports and then we will see.

On some machines (generally) SATA-5/6 are reserved for Optical drives. Sometimes the BIOS has an option about it.

There is only 4 HDD bays anyway. Can you limit HDD/SSD to SATA-1/4, and use 5/6 for opticals?

Size 4gb might be limit for drives on Aurora-R4 ... even though it's not suppose to matter if the larger drives aren't bootable. Sometimes, HDD maker has translation software to work-around BIOS glitches.

White 1 2 for non-boot / optical drives >< black 3 - 6 for boot drives