giacam:
The confusing part is that my understanding is that Digital write (HIGH) should mimic the 5V out put.
You need to understand the difference between a signal line and a power line.
Signal lines use voltage(s) and/or current(s) to carry information (analog of digital), but
operate usually at very low power levels (less is better for signals, since its waste power).
A power line carries power.
Never confuse the two.
The Arduino pins are logic signal outputs - digital, 5V. They are rated for an
absolute maximum of 40mA (normally logic signals are much less than that,
a couple of mA is common).
A motor needs power. Most small motors take 1A or so, larger motors upto dozens
or even hundreds of amps.
Also motors are inductive - they can kick out huge(*) voltage spikes if protection
circuitry isn't employed.
You are likely to have already damaged or degraded that output pin on your Arduino, I
would test whether it still works with an LED+resistor. Test it both sourcing
and sinking current (LED to GND and then to Vcc, with resistor in series).
To control a motor with an Arduino you have a motor driver / controller board or
shield, the control signals go from Arduino to the motor driver, the high-current
outputs of the driver go to the motor (and a power supply for the motor is also
connected to the board - best if separate from the Arduino supply although the
grounds must be commoned).
For DC motors a motor driver is typically an H-bridge circuit - 4 high-current
switching devices in an H-configuration. Typical Arduino motor shields employ
2 H-bridges but are often limited to about 1A.
MOSFET H-bridge boards are available for higher current - Pololu have a reasonably
comprehesive range.
(*) The voltage is whatever is necessary to keep the current flowing - usually
destructive to semiconductor devices. Diodes are typically employed to provide
an alternative route for the current.