As I wrote in the title, I would like to connect a 5V fan to the USB 3.0 port of my RB5009 (non-poe) because that’s would be the only way I have to power it easily.
Can it be done? Ofc the fan will not draw more than 5W (iirc USB 3.0 max wattage is 15W)
From specs 1.5A max:
https://mikrotik.com/product/rb5009ug_s_in
USB slot type USB 3.0 type A
Max USB current (A) 1.5
5V*1.5A=7.5W
If you need more (and you don’t need to switch it on and off programmatically) you may want to use a Y cable on the power supply line (and a buck converter for lowering the voltage if needed).