2nd M.2 on B550 not identified in BIOS/UEFI - Please help

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I've just finished my first proper build, and this is the last hurdle I am stuck on - any help greatly appreciated, I've scoured the web but cannot identify a fix. I've posted a similar summary to different forums to little avail - in my search for fixes I found several people raising similar issues here with more support, so I am coming here to see if anyone has the expertise or experience to offer advice.

I have an Asus Rog Strix B550-E gaming motherboard - which according to the manual supports the following:

'M.2_1 slot supports PCIe 4.0 x4 and SATA mode Key M design and type 2242/2260/2280/22110 storage devices.

M.2_2 slot supports PCIe 3.0 x4 and SATA mode Key M design and type 2242/2260/2280/22110 storage devices.

When M.2_2 slot is populated, SATA6G_5/6 will be disabled.'​

In the M.2_1 slot, I have a Sabrent Rocket 4 Plus M.2 PCIe Gen4 x4 NVME SSD. This is working fine, and is currently carrying my OS.

In the M.2_2 slot, I have a Sabrent Rocket M.2 PCIe Gen3 x4 NVME SSD. This drive is not being found in my BIOS/UEFI (or in Windows as unallocated space).

When I place the Gen 3 drive into the M.2_1 slot, the PC does not post.

When I place the Gen 4 drive into the M.2_2 slot, the PC does not post.

I have updated my UEFI to the latest non beta version (which is also the latest version).

I have entered the UEFI/BIOS in EZ and advanced modes, but don't seem to have any relevant settings to change to get the drive to show.

I do not have any devices plugged into any of the SATA ports - so the disabling of SATA6G_5/6 should not be an issue.

I am at the diagnosis point that either the motherboard M.2_2 slot is faulty, or the drive is faulty. Out of pure frustration, I have ordered a different M.2 PCIe Gen3 x 4 NVME SSD (Western Digital Black SN750 500GB M.2-2280 SSD) to see if this makes a difference. However, in my search and seeing how many people have had difficulties with M.2 drives not showing in the BIOS, I feel like its just something I am getting wrong in set-up (and this seems bolstered by the PC not posting in certain configurations of M.2 layout).

Please give any advice you might have (even if it seems obvious and like something I would have tried already). Everything with my first build is perfect apart from this issue so really just trying to resolve it.
 
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Thanks for responding @Hakanese, I thought I had tried all of these steps, but I might not have done them all at the same time/in the right order - I will try again right now and report back shortly.
 
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I had a similar issue when I went m.2 for the first time this year, turns out I had it in the wrong slot:D
 
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Alright so, whilst opening up the case (again), I cleared the CMOS (again) by removing the battery - this time leaving it for about 10 minutes. Upon rebooting - the drive is now recognised! I can't believe I've been bashing my head against a wall for 2 days trying to figure this out and now it just popped up. I am hoping that there will be no further issues when re-enabling DOCP.

Thank you for your suggestions - if I now wanted to transfer my OS to that drive instead, do you have any recommendations to ensure I don't run into further issues? (e.g. should I allocate that unallocated space in disk manager now, then remove the PCIe Gen4 drive and do a clean windows install with only the secondary M.2 and the USB for installation connected?).

You have saved me many a headache - other than the realisation that despite thinking I had done everything right, I am just an idiot.
 
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After reaching this stage and trying to allocate the space in disk manager, I am now getting the message 'A device which does not exist was specified', and whilst I can see it in Disk Manager, it says 'Unknown, not initialised'.

Is this indicative of a fault with the drive? I have a different drive arriving today so will test that and report back if I get a resolution. Any advice on the error message and the drive disappearing from BIOS again, and what that could mean, would be helpful (no changes made in BIOS between it appearing, and subsequently not appearing again).
 
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New drive installed - and worked straight away! Must be a faulty drive, all this troubleshooting for nothing. What set me off on the wrong foot is the other m.2 (PCIe 4.0) not working in the lower M.2_2 slot (system wouldn't post) - and as they are backwards compatible it made me think the issue was the motherboard slot - however that does not seem to be the case.

Huge relief - thanks for the patience and advice @Hakanese - now just to return the faulty drive.
 
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