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That was put on there for low power devices. It is mainly used as an auxiliary power source for lower power consumption on SSDs when available.
 
3.3v line is probably for newer technologies. No known HDD uses the 3.3v line but maybe SSDs use them.

SATA power plugs have many pins because they need multiple pins for power because one pin is too small. I think each power wire uses 3 pins or something like that. Also a few more pins are used for hot-plugging technologies.
http://superuser.com/questions/98274/why-are-there-so-many-pins-on-a-sata-power-connector

why not just keep the legacy 4 pin Molex like some of the early SATA HDDs do?
 
Because it is large, held in only by friction, and not good for hot swapping.

a SATA Power is larger; Thinner but wider and itself held in by friction. I actually have had many SATA HDDs stop working because the SATA cable or the power cable wiggled loose with vibration of the HDD; Don't have that problem with IDE connectors or Molex plugs

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New technology
Hot-plugging capabilities
Slim
Universal (same connector for 3.5 and 2.5 form factor)

i am sure there are plenty of PSUs in use that do not have a SATA connector, hell the only PSU i have that has one is inside of my HP desktop, a modern SATA HDD still having BOTH SATA power and Molex power would make a better fit for universal application in legacy systems (such as PowerMacs) :D
 
a SATA Power is larger; Thinner but wider and itself held in by friction. I actually have had many SATA HDDs stop working because the SATA cable or the power cable wiggled loose with vibration of the HDD; Don't have that problem with IDE connectors or Molex plugs

The SATA power cable is designed to have a thinner profile. They are also designed to have a clip on them. Cables not having a clip on them are out of spec and prone to the problems you have noticed. Molex cables can sometimes bind to the drive resulting in a very difficult removal that may result in damage to the cable or the drive. Most modern, even going back five years, power supplies have SATA power cables and no Molex cables.
 
The SATA power cable is designed to have a thinner profile. They are also designed to have a clip on them. Cables not having a clip on them are out of spec and prone to the problems you have noticed. Molex cables can sometimes bind to the drive resulting in a very difficult removal that may result in damage to the cable or the drive. Most modern, even going back five years, power supplies have SATA power cables and no Molex cables.

yeah, the thing I don't like about molex is that they are hard to remove and don't have a good "click".
 
The SATA power cable is designed to have a thinner profile. They are also designed to have a clip on them. Cables not having a clip on them are out of spec and prone to the problems you have noticed. Molex cables can sometimes bind to the drive resulting in a very difficult removal that may result in damage to the cable or the drive. Most modern, even going back five years, power supplies have SATA power cables and no Molex cables.

the 500W PSU i have in my HP i bought like 2010 it has 3 SATA and like 5 molex as well as a LP4 and a 6 pin PCIe. Of course now it has 2 LP4 connectors and 6 Molex because i have had to replace the Molex on most of it.

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yeah, the thing I don't like about molex is that they are hard to remove and don't have a good "click".

i find that to be more common on the 90 degree molex like Apple uses than the straight molex connectors
 
Budget box PCs from five years ago use even more budget power supplies designed from an even longer time ago.
 
Budget box PCs from five years ago use even more budget power supplies designed from an even longer time ago.

may be true but back in 2010 i paid like 50 bucks for the PSU because i wanted one with 2 fans one in front and in back and that was back when 2 fans in PSUs were just becoming a thing so it wasnt cheap Not at all happy with the low quality of the Molex though since most of them shorted out resulting in me having the splice in new
 
Multi fan power supplies have been around since the 80's. $50 is still rather low budget for a power supply.
 
Multi fan power supplies have been around since the 80's. $50 is still rather low budget for a power supply.

may be for most people but when you only make $200 a month $50 is rather expensive.

When i manage to get a SATA card for my mac i will most likely chop off the molex in the MDD (Leaving one for my USB 2.0 card) and wire in SATA connectors. i have yet to ever have a locking SATA power connector i have a few single or dual locking SATA cables though. It still puzzles me what if the MDDs PSU could handle in theory 12 SATA HDDs at 360W why my USB card locks up while transferring files when i have just 4 HDDs installed yet the problem goes away with just 2 HDDs
 
The USB card is likely suffering from a dirty power feed or interference caused by some of the drives.
 
The USB card is likely suffering from a dirty power feed or interference caused by some of the drives.

perhaps the homemade Molex extension cable? just took a Molex off a dead fan chopped it in two and added wires to it to make it longer than did a wire "hook" on both ends and wrapped it in electrical tape.
 
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