If the system boots normally without the drive, the most likely issue is a faulty drive (if it's a Seagate drive, that scenario becomes even more likely). Have the drive replaced (or try another drive).  If another drive does the same, the other possibility is a damaged connector or connector cable.  Did the cable come direct from Dell or is it third party?  
Hi,  I have a Dell xps 15 9550 with a SSD 255gb so I need to upgrade it.  I have room for 2.5 disk so i bought a new 2Tb HDD and the connector.  The problem i found is that when powering on, the power light blinks and the computer remains off.  I formated the hdd in other pc, and result.  I tried a smaller hdd (256gb), and result.  I tried a new cable, same result.  I tried changing boot sequence,  same result.    Any idea on how to solve this issue? 
XPS 15 9550 2nd Drive