I just bought a Seasonic CORE-GM-650 to replace a xpg probe 600 ,and this is my first time dealing with a modular psu.

I'have 5xhdds. I'm planning to add another hdd to my system soon and I may use a molex to sata cable to connect a fan hub controller. There are 2 sate cables, 1 pci-e cable ,and 1 molex cable included in the box.The three 6pin connectors on the PSU's end are meant for SATA devices or any other device that needs a 4pin molex connection.
What sort of devices do you have to work with apart from the motherboard and (assuming)discrete GPU?
This cable goes in there. You can put either SATA cable or molex cable (whichever you need).I just bought a Seasonic CORE-GM-650 to replace a xpg probe 600 ,and this is my first time dealing with a modular psu.