Mostly we use HDD less than 6TB capacity, so mostly the HDD mounting in mini pc is designed for less than 6TB 3.5 inch HDDs, Screen design is not ok for HDDs up to 6TB at all. If you are going to buy mini pc chassis for DIY computer, make sure understand it in advance.
Here is Mounting dimensions for less than 4tb 3.5 inch Seagate HDDs:
And here is Mounting dimensions for up to 6TB Seagate HDDs:
Here show HDD details: