Modifying one of the u.2 slots is not feasible, the backplanes are in pairs If you add the kit on the back, you gain 4 slots... is practical if you foretell to need to add more storage in the future. If you, instead, really need just one extra hdd ( even long term ), and if the front 5.25" bay is not being used, you could buy a 3rd party 5.25" to 3.5" adapter and put there the hdd ( internal cabling would be on you to arrange )
We have this computer: [URL] It has 4 HDD slots : [IMAGE] Slot 1 was empty. Slot 2 has 512 GB M2 SSD (flexbay) Slot 3 has 2 TB HDD. Slot 4 was empty.   I added two more 8 TB HDD to empty slots. HDD in slot 1 is not working. It is not showing on Ubuntu 18 x64 OS. I swapped Slot 1 and Slot 3, then old HDD (2TB in slot 1) is not showing but new HDD (8 TB in Slot 3) is working. So nothing working in slot 1. But the light be on in Slot 1 HDD: (red arrow in the picture) [IMAGE] What is the problem, is slot 1 is broken or is not supported HDD? (maybe only run flexbay SSD?)   Question 2: Manual says computer can have 8 pieces HDD: [IMAGE] Where are others 4 piece HDD slots? I could not found.  
HDD Slot not working in 7920