How many hardrive can the motherboard power

[savvy2]

his PSU has no such cables(hdd)

it is a HP non standard PSU, HP custom.

so all HDD  power comes from mobo only.

unless upgraded, as told in my first post

as all mine are. here. hp and dell all have ATX 750 PSU)

[savvy2]

OP   (let me guess no HP cables are present at all) I can not see your PC, (photos work here)😁

not sure what you have on your bench a NAKED  MOBO?😋

THERE BOOK  ON THIS PC tells what all connectors do on the MOBO< everyone (not factory test jacks mfg)

they do  not document pinouts on all nor current limits on ANY jack.

HP  does not sell DIY kits ever (try ASUS) but does sell PCs and does have cables that work as  delivered new.

you will have more troubles when you turn it on and the BIOS finds all those HP accessories missing, oops.

OEM PCs may even be very upset even using the wrong fans.  PWM to RPM rules fail oops wrong fans installed.

even 2 fans failing or front panel missing.

the PSU is only  240 watts, why are you worried about currents?  no goal told by you, and END GOALS MATTER and X10 that for DIY anything,.

yes the mobo has onboard VRM , more that real DIY MOBO haved,

VRM 5volts and more, voltage regulator models, and yes the mOBO creates 5vdc and sends that to the hdd ports.

now I  will show you the manual, that is on hp.com for your PC on YOUR support page

google HP prodesk 600 manuals ,1st hit.

in the system guide the mobo is called SYSTEM BOARD.

https://support.hp.com/us-en/product/hp-prodesk-600-g1-tower-pc/5387438/manuals

I can only assume this is full sized PC not some SFF or the like,  HP makes them not in one size. nor a MINI !

TOWER PC (big)

Page 97 here  answers where jacks are  and what they do. "TOWER" only !!! 

http://h10032.www1.hp.com/ctg/Manual/c04331099

ID 13 is  SATAPWR jack clearly stated in the manual, HP is one of the best non DIY makers of PC, for documents as you can see. (not as good as ASUS or Gigabyte but they are DIY makers) so no comparisons made by me ever, OEM !=DIY

there are 20 jacks. on mobo.

I think I answered all your questions and with DIY there well be 10 more I AM SURE. 😍

like where is poweron button wiring, oops, #5:?

[savvy2]

https://support.hp.com/us-en/product/hp-prodesk-600-g1-tower-pc/5387438/manuals

the manual is free

pg 97 on the service manual

tells what each of the 20 jacks do.

[yota_4wd]

So tu reply to your answer 

The motherboard photo isn't mine i don't have access to it right now but it's the same 

IT is indeed the full size tower 

And my power supply is 320 W max so maybe not the same your thinking about

I already snooped around this datasheet or more user manuel but did not found my goal and ,

My goal was too have the current output of the satapwr but like you said it seems to not be public so i will do more resarching on the board to see if the mobo is only a pathrough or if it does some switching for the 5 V and i was planning for 10 HDD max for now and my wiring was to double the sata power avalaible and to make them longer as i have done some dremel surgery to add other drive bays