Using hard disk motor as generator
A HDD motor usually is brushless, i.e. it has no commutator. 
Basically, it consists of three coils arranged around a magnetic rotor. There are three terminals to power the three coils and one common current return terminal:



You can drive the motor by a 3-phase AC voltage, and when you turn it by hand, it will generate a 3-phase AC voltage.

Here is a youtube video of someone who measured the voltage on all three terminals while spinning the motor by hand. This screenshot from the video shows a wonderful 3 phase voltage:



As you need DC, you could use three rectifiers:







simulate this circuit &ndash; Schematic created using CircuitLab

or six diodes, as @jms pointed out:







simulate this circuit

However, I'm not sure how powerful those motors are, or how much power you get out when using them as generator. @JRE's experience (see comment) is that the power is very low. This may not be enough for your experiments, but if you want to try, choose diodes/rectifiers with low voltage drop (schottky diodes) to reduce the losses in them.