SATA power splitter.  I believe this is what you're looking for?  [URL] 
Dell Precision T5810 / Windows 10/64Bit I need to add a third drive, a 2.5" SSHD. To do this, I would need a power source. The power source beneath the optical is not the right size. Where do I get a power supply source for this third drive? Thanks.
Need A Power Source For A Third Drive