Why can't my PC read my second hard drive?

Episode 1329 (16:03)

Mary from North Dakota
HDD

Mary added a second hard drive and it won't spin up. Leo says it's likely that her power supply isn't rated to drive two spinning hard drives. She could replace the power supply first, or try an SSD, as they take up less power. If those two don't solve the issue, then she's looking at a motherboard issue, but that's unlikely.

(Image By Evan-Amos (Own work) [CC BY-SA 3.0 or GFDL], via Wikimedia Commons)