# hdd-only power supply unit possible?

## xibo

Hello

I'm running a dedicated file/boot server which used an excessive amount (18) of SATA disks in a software raid. Since i extended the raid from 16 disks, the server is "switching off" every now and then, even though the 2 new disks work perfectly in another system that has the same sata controller. My thought is that the problem is the servers' 550W power unit to be too weak for so many devices.

As the cabling is already a mess, i was wondering whether it is possible to connect the HDDs on an individual power supply unit. The main problem is that the extra power unit has to be signaled to startup/shutdown with the system ( do i even have to power cut HDDs? ) while i have no clue which cable/string is used for that purpose.

The other problem would be, that eventually the SATA data cables would act as ground/balance between the main PSU's and the support PSU's circuits which might damage the controller cards or/and hard drives ( i had ethernet cables acting as ground destroying switches/hubs before, though the endpoints were in different buildings with distinct power circuits ).

So i wonder, is it possible to use an ( or multiple ) additional power supply unit(s) to feed HDDs, or must they be supplyed by the core systems' power unit? If former, which strings need to be _triggered_ for a power unit to startup/shutdown?

Alonso

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## NeddySeagoon

xibo,

You can use a separate PSU for some or all of your hard drives. 

You must connect the black wires of all the PSUs together so you have a common return or as you say, the data cables will do the job and this is a very bad idea.

You may need a minimum load on the 5v or 3.3v of the extra PSU to make it run. Many PSUs will not run on no load on that converter.

Hard drives use 5v but not 3.3v.

The PSU is turned on by connecting its PS_On# signal to ground, very briefly, its turned off the same way.

In the 20 pin ATX motherboard connector, PS_On# is the green wire between two black wires, next to the catch.

You should be able to connect these two green wires together, so both PSUs operate from the same switch.

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## eccerr0r

It depends on the design of the PSU whether it will run without a load on the +5 / +3.3.  I've seen some PSUs that use +12V as their feedback line (and leave +5 open loop) and thus will regulate fine with no load.  But on the other hand if it does use an unloaded/underloaded output as its feedback loop, prepare for some very ugly things to happen to your downstream devices... I fried a HDD trying to use an old IBM AT PSU with no load other than the HDD... it let the magic smoke out of the motor driver  :Sad: 

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## krinn

woudn't it be just safer to just buy a stronger PSU ? there are upto 1000W ones now.

You can also brought some disk case, that handle the extra disk with their own psu and cable and that can be add to your main computer with an external connector.

Or a real case with a motherboard in it just to be power on/off that provide all your hdd the supply, this way you just have to switch it on to provide all your hdds power, and if off, your main computer will just complain it cannot find any hdd while booting.

I suppose not what you were looking for at first, but really, do you really need to auto switch on/off your hdds for a server that have all those hdds ? I mean, it's not really a desktop usage, and i don't see the point to switch it off/on like a desktop computer could be.

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## Mad Merlin

I'd side with krinn on this one, it's hard to even find modern PSUs that supply less than 550W (with well into the quadruple digit wattages being easy to come across). Unless of course you don't mind risking frying 18 drives and restoring from backups. (How do you backup 18 drives worth of data anyways?)

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