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On the drive that is not showing:

Click the Start Menu and type "create and format hard disk partitions" and press Enter.
Look at the lower section and check that all of your installed drives are showing.
If they are all showing, make sure that all of the disks are set to "Online"
If there is one that is set to "Offline", right click on the title box and set the drive to "Online"

After this you should be able to use the drive.

If the drive is not showing in "Disk Management", try the drive in another computer or in an external HDD bay to check if it works.

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This computer is new enough that any SATA or SAS drive that you plug in should work.
The Dell spec sheets lists compatibility with up to 5x 12TB SATA drives.  Larger drives will work (12TB was the largest available when the spec sheet was published)

Hello, 

Thanks for your explanation. I am using Ubuntu 18.04 and searched disk and also in command line but still one HDD is not showing at all. Is there anything that I can do? or need to check whether the slots has problem. I checked HDD in external device and it worked well. 

I am using xeon w-2295. Can I use 4TB HDD here in 4 slots (front flex bay).

Thanks