Try this post at Community Spiceworks and this Dell support page. The only other thing I can think of is that HDD which are large like your 12TB HDD are not compatible.

Could you clarify.

Do you mean that the Solid State Drives (not the NVME ones) are being detected in Windows, but that Windows is not detecting the same Flexbay locations when they are mounted with Hard Disk units?

To clarify my above comment.  The T7920 has two bays for NVNE but I use only one bay with a NVME SSD 1TB.  The second NVME bay is currently empty.

2 Bronze - you are correct and I am running Windows 11.

The T7920 has the following:

- one NVME SSD 1TB detected and is working fine

- two SATA SSDs detected and are working fine.

- four SATA HDDs non is detected.  The four SATA HDDs range in sizes from 2TB to 12TB.  They all work fine on my older T7500 Tower workstation.  I am upgrading and moving the four SATA HDDs from a current T7500 workstation to the newer T7920 workstation.   

You already confirmed all the drives and flexbay are working then it just leaves a basic common user error, the drive was not plug in to the backplane properly.

SATA connections should be fine as the SATA ssd’s are working fine. This could indicate the power issue because hdd needs 12V vs 5V of ssd’s. 

Rain_HH

The power supply that comes with the T7920 is not standard with enough voltage to support 2-bay NVMe and 6 bays SATA HDD?   Is there instruction that I can find to verify the voltage to each bay?

I installed a 4-bay expansion kit on the rear of the T7920.  The kit comes with a power supply cable that split it into two connections: one for the front 4-bay and the other for the back 4-bay.  Could this splitting reduce the voltage to all 8-bay cause only SSD and small NVMe to work only and not the higher voltage SATA HDD?

Thank you both for your suggestion.  

As per the update, you will need to go to the place of purchase as the tag is not supported by Dell but by the reseller.

Mine has an1400W PSU and is fine. Not sure what is your PSU wattage. However, I am more concerned with the connections at the back of your unit than the PSU itself. If the connections are fine, there should be something wrong with the PSU. Also it is worth to double check the BIOS to enable all the HDD's if they are not checked the BIOS.

Precision 7920 Tech Guide.pdf 

1100w PSU is definitely more than enough for the HDD's, unless the GPU is a million watts.

I like @Rain_HH's idea of checking BIOS.  If those HDD's are already checked off, uncheck them and click Apply.  Then recheck them and click Apply.  I've had to do this sometimes when adding hardware on other Dell's.

If still no luck, with the drives installed, click Update in Device Manager.

I take it the HDD's are either blank, were used for storage, and don't have an OS on them, and not part of an old RAID setup.(?)  If it helps, HDD specs are on og. 17 in the Tech Guide.  3.5" 12TB Enterprise HDD is listed.

Dell Precision 7920 Tower Owner's Manual