Hello!
I have a Raspberry Pi 3 B+ with a power supply 2.5A, 5.1V. I plan to buy a Maxtor 4TB portable HDD, not powered.
My question is, what do you think, do i need a powered usb hub, or can is use it just with the RPI? I use the RPI for torrent and ownCloud.
Depends on the output capabilities of the USB port…if enough to power the drive then yes…if not (for stabilities sake) then provide external power (I had to do that with one of my USB to SATA adapters [cut open, splice into a USB cord] then tie into the pi)
You definitely need external power if you think it might be a bit underpowered and you have dropouts
https://raspberrypi.stackexchange.com/questions/340/how-much-power-can-be-provided-through-usb
https://hackaday.com/2015/04/06/more-power-for-raspberry-pi-usb-ports/