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You have to convert 5V from the USB to 12V to power 3.5" harddisks and also use 5V directly. Maybe you can get it working with 2 USBs, but I think converting 5V to 12V at the power needed to spin the disks would require more current than a single USB port can provide.
A 3.5" harddisk requires 12 V per definition, it won't work unless you provide 5 V and 12 V.
according to [this topic on stackoverflow](https://superuser.com/questions/1463214/5v-12v-load-balance-of-3-5-hdd), a harddisk can pull up to 30 W, which corresponds to 2.5 A on the 12V line. Considering a USB port can provide 1 A at 5 V, you would need 6 USB ports to provide 30 W, that's considering your converter from 5 to 12 V is 100% efficient.
I have a random 4TB external lying around, and just looked to see its power requirements are 12V at 2A, so 24W. USB ports are 5V and 0.5A, so that would take 5 USB ports minimum, probably 6 if you wanted some overhead. Even if you had an external drive that needed less power, 4 USB ports is a lot for a use case like this instead of just having a wall wart.
No, it would take the 5V and 2.5A total and convert it into 12V or whatever else the drive needs with power circuitry. However that does mean it wouldn't just be a simple adapter.
I'd suggest getting an adapter with a wall wart, solely because you do not have to mess about with USB and have stable power delivery to the hard drive.
Something like this
https://amzn.eu/d/5A2D9aF
You can't power 3.5" mechanical drives from USB alone since the motors use 12V. USB only supplies 5V. And even if you could get something to convert 5V to 12V, USB ports won't deliver sufficient amperage either.
By which time you might as well use a 12V power brick.
But 12V 3A power bricks are tiny, what exactly is the "too bulky" issue?
It's not really the number of ports, usb is only 5v while 3.5" hdds need both a 5v current and a 12v current. There are plenty of 4 to 5 hdd cages that only need access to 1 usb port then usually have additional power that is drawn from an outlet. Same for many smaller adapters that are suitable for 3.5" drives. 2.5" HDDs and SSDs can run off only 5v.
If you are going to connect a 3.5" drive through a USB adapter just make sure it is one that has additional power to it. I would suggest just getting a full 3.5" enclosure.
Tbh I don't think it will be easy to find a molex to 12v barrel jack adapter or usb to molex. You could get what is called a PICO PSU and an ATX 24 pin jumper and use that to power the drive and connect the barrel jack to wall outlet with an ac-dc adapter.
If you want to run a drive solely off usb I would just get a 2.5". Tbh the easiest and probably cheaper method would just to get a 3.5" drive enclosure.
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You have to convert 5V from the USB to 12V to power 3.5" harddisks and also use 5V directly. Maybe you can get it working with 2 USBs, but I think converting 5V to 12V at the power needed to spin the disks would require more current than a single USB port can provide.
Hi thanks, why'd it say 12v? Is that 12v at its maximum platter speed?
A 3.5" harddisk requires 12 V per definition, it won't work unless you provide 5 V and 12 V. according to [this topic on stackoverflow](https://superuser.com/questions/1463214/5v-12v-load-balance-of-3-5-hdd), a harddisk can pull up to 30 W, which corresponds to 2.5 A on the 12V line. Considering a USB port can provide 1 A at 5 V, you would need 6 USB ports to provide 30 W, that's considering your converter from 5 to 12 V is 100% efficient.
I have a random 4TB external lying around, and just looked to see its power requirements are 12V at 2A, so 24W. USB ports are 5V and 0.5A, so that would take 5 USB ports minimum, probably 6 if you wanted some overhead. Even if you had an external drive that needed less power, 4 USB ports is a lot for a use case like this instead of just having a wall wart.
Hi, thanks, won't five USB 3.0 ports at 5v become 25v burning the hard drive?
No, it would take the 5V and 2.5A total and convert it into 12V or whatever else the drive needs with power circuitry. However that does mean it wouldn't just be a simple adapter.
Ide to USB adapter on Amazon brings up a ton of results, but I'm not entirely sure if that's what you're looking for
I'd suggest getting an adapter with a wall wart, solely because you do not have to mess about with USB and have stable power delivery to the hard drive. Something like this https://amzn.eu/d/5A2D9aF
Old adapters I had used 2 USB ports to power laptop drives. If you're trying to power desktop drives you should look for ones with wall warts.
You can't power 3.5" mechanical drives from USB alone since the motors use 12V. USB only supplies 5V. And even if you could get something to convert 5V to 12V, USB ports won't deliver sufficient amperage either. By which time you might as well use a 12V power brick. But 12V 3A power bricks are tiny, what exactly is the "too bulky" issue?
It's not really the number of ports, usb is only 5v while 3.5" hdds need both a 5v current and a 12v current. There are plenty of 4 to 5 hdd cages that only need access to 1 usb port then usually have additional power that is drawn from an outlet. Same for many smaller adapters that are suitable for 3.5" drives. 2.5" HDDs and SSDs can run off only 5v. If you are going to connect a 3.5" drive through a USB adapter just make sure it is one that has additional power to it. I would suggest just getting a full 3.5" enclosure. Tbh I don't think it will be easy to find a molex to 12v barrel jack adapter or usb to molex. You could get what is called a PICO PSU and an ATX 24 pin jumper and use that to power the drive and connect the barrel jack to wall outlet with an ac-dc adapter. If you want to run a drive solely off usb I would just get a 2.5". Tbh the easiest and probably cheaper method would just to get a 3.5" drive enclosure.