How can I power a hard drive so I can use it with my Pi without using a powered usb hub? Can I use a small power supply?
Theoretically, given an adequate power supply, the Pi3 USB can supply 1.2A, other models vary - See Raspberry Pi Power Limitations
I have verified the current capabilities with static tests.
Hovever in practice I often find USB hard drives work unreliably; I suspect this is due to the high inrush current when the motor starts.
Many newer high capacity USB3 drives draw quite high currents, even those which claim to be USB2 compatible.
There is no definitive answer to this question, and it seems to depend on the actual drive.
While I have successfully used a variety of drives, to be on the safe side I usually use a powered hub.
If your drive is connected via the USB ports then it will have access to 600mA from the Pi. You should check the drive to see how much current it will draw. If it's a power efficient 2.5" drive you might be able to get away with it. I seriously doubt you could use a 3.5" drive without another power source.
You can hardwire a 5v supply into the usb cable of the device you are using, but give a ground lead from that supply to one of the RPi's GPIO pins. I wouldn't recommend this though. Powered USB Hubs are pretty cheap anyways.
Hope this helps :)
If you really don't want to buy a hub, then you could either modify the hard drive, to be powered from an external 5v power supply, or use one, that already has external power.
This may be a duplicate, as it pertains to power a USB device with more amperage.