Can I use a transistor to turn on a 10/12v supply with a USB cable, like this?
I would do something like this. :
The slow speed (by self pwm at 50%) is optional, but I decided to include it



simulate this circuit &ndash; Schematic created using CircuitLab
The above is for always-off pwm fans. For free running fans when the pwm lead is not used, I used the fallowing on my raspberry pi pwm fan:



simulate this circuit