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As you can see from the case, it does not have enough room for you to add another HDD. It is a desktop anyway, you can always use external HDD to store data.
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I clonned windows 10 OS to SSD 2.5 and the PC boot up just under 20 secs. In order to add 3.5 HDD that came with the HP 510-p030 and use as extra storage, I installed 4 posts 3/4" and 4 screws 6-32 from local hardware store to make the 3.5 drive offset from the SSD so I can plug in 2 cables (1 power and 1 sata removed from DVD drive), reverse the white color sata cable so the straght end can plug into the 3.5 HDD and the 90 degree right angle goes to the main board. Hope this will help.