Hello!  If Dell sent you a replacement HDD you are obligated to send it back as the old HDD becomes property of Dell.  They send it back to the manufacture for diagnostics and testing for future products.  You technically can keep the old HDD, however Dell will charge you for it and it may flag your account in which they may request any future repairs are done by a technician onsite if you have that warranty package, or you may be forced to send your computer in for warranty assistance if it comes up in the future. 
the original HDD, which was replaced with a new one as a result of repairing  a software problem reclamation....problem was with a WINDOWS 10 installation, without aquiering the 100% finalization of WINDOWS UPDATES .wich ended such as a dead end..but if the HDD is priceless, why should i give it away, and one reason why shouldn t i want to sent it back...I JUST DON´T WANT TO :) thank you for answering, AND HAVE A NICE DAY, please:)Nowak
I would like to know, why should I send back the original HDD,