[NAME], You will not be able to connect 3.5 inch HDD on a 2.5 inch HDD slot as it will not fit in.  R740 with 16 HDD slot only support 2.5 inch HDD.
Hello everyone .  I would like to add additional space to my R740  . I need big HDs (The more the merrier) . On my system I currently have 2.5 HDD only . I looked it up on DELL's website and for 2.5 the biggest one is 2.4TB . However for 3.5 is 14TB . Out of 16 slots on my server I have left 4 available . Can I put there 14TB 3.5 HDD each ? Or the system must have the same size (2.5) for all the drives ? My assumption it won't matter but I figured I'll make sure before ordering .  The 14TB disk is 7K RPM while the 2.5 I have are 10K  .This server is being used for deep learning so I'm also wondering if the read/Write performance will decrease dramatically. Thank you for your help .     Thx . 
2.5 HDD with 3.5 HDD on PowerEdge R740