Welcome to the community, @Garett_1!
I'd recommend you check the connections inside your PC and make sure you have mounted the HDD properly (both to the motherboard via SATA and to the PSU via the Molex cable). I'd also suggest you try plugging it with a different SATA cable and check how it will get recognized if plugged to a different SATA port as well. If you have access to another system, I'd also suggest you check how the drive would behave from there. This will help us determine the source of the issue, hopefully.
On your system, you should also check the boot priority and try
resetting BIOS to see if that will help you with the proper recognition of the boot drive. Try switching between the SATA configuration modes (AHCI:IDE and vice versa).
Let me know how it goes. Good luck!
SuperSoph_WD