I would like to connect two external 2.5 HDDs a Pi 4. Can the Pi 4 power two of them at once?
This is the model I have in mind: WD 2TB Elements
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Not without voiding your warranty. An RPi4 can deliver a maximum of 1.2 A to all the USB ports together. This is done to protect the USB-C connector which is rated for 3A maximum.
The disks you have in mind consume up to 1 A each. Without touching to the Pi, you'll need to use a powered USB hub or a bigger-capacity single disk. If you don't care about the warranty of your Raspberry and you're powering it externally, you can bypass the current limitation by connecting the 5V GPIO pin to the Ubat pin of USB directly (check out the semi-translucent copper wire):
P.S. A small 5V capacitor between the USB 5V and nearby GND (I used 300 uF between the same point where the wire arrives, and one of the 4 bigger solder joins) greatly improves the stability of the USB w.r.t hot-plugging new devices. Otherwise hot-plugging a second HDD may produce a voltage dip which reboots the HDD that was already connected.