I’m about to put together my first NAS. My case is a Jonsbo N2, and I have 5x 8TB HDD’s which connect to the backplane inside the case. The backplane is powered by 2 molex ports, pictured here. My 650W PSU came with a single molex cable that daisy-chains 3 molex connectors, like in this picture, and I want to use it to power all five HDD’s.
Question: is it safe to plug in two of these molex connectors to power all five HDD’s on the backplane, or it is advised to use two seperate molex cables?
Check the specifications of your PSU, but if the rails are designed to safely feed 3 drives, they should handle 5—and you don’t have much choice anyway.
Seems your case requires an SFX PSU which should give you 3 peripheral connectors, unless its an older model, so I’m wondering what the other 2 are used for? In my case, I connected 2 Molex to SATA adapters to one Molex cable and you might consider a SATA to Molex arrangement if one of those peripheral connectors is used for only one SSD.
I’ll start in short by saying that you will be fine. If you want to understand how I can be so sure, then read on with my apologies for the length. 
I would estimate that if your cables are the same as the ones in your photo, that cable is typically 18AWG gauge. Check the markings on the wires to confirm.
If it is indeed 18AWG, you’ll be OK, as the rated DC current for 18AWG cable is around 14A, from memory. You haven’t specified which drives you have, so I will go with the typical peak of 2A for an active mechanical HDD such as a WD Red Pro. The higher amount of current will be drawn from the 12V line, as it is the one that deals with the mechanical components, which draw more power than the 5V logic circuitry itself.
That peak current wouldn’t last very long anyway, since it’ll be generally just for start-up and then things chill out when it’s just running. Still, I’m going to run with that anyway for the sake of being conservative in our calculations.
So, five drives, 2A consumption each, that’s 10A in a 14A wire.
Again, that does not mean you will draw 10A constantly after start-up. I estimate it would go to less than half that, based on the same Red Pro’s average power requirement going down to 8.4W when active (let’s round off to 9W). I have no specification data on how that power is divided between the 5V and 12V lines but I’ll estimate about 25% for the 5V logic and 75% for the mechanical.
In which case, that divides the 9W into 2.25W for the 5V logic and 6.75W for the mechanical.
That makes your total current draw a total of (2.25 / 5 * 5) = 2.25A for the 5V and (6.75 / 12 * 5) = 2.81A for the 12V.
At around 3A per wire, for a wire that is rated to handle 14A DC at typical home operating environments, you’re fine.
Thanks for your help guys. That’s a fantastic answer @artstar, I appreciate all the small details. The drives are actually WD Red Plus and the PSU is a high quality Seasonic Focus SGX-650, so I trust your assumptions are spot on.
Yes.
Every HDD uses max 20 watts at startup and motherboards start multiple HDDs in sequence to avoid all of them starting at once.
After that, each HDD only uses 10-15 watts (idle and max).
