Since your motor is from a 5.25" floppy disk drive, perhaps you'll find it useful to examine the circuit that's used on the type of FDD from which you obtained it. That should give you some guidance on how to drive the motor, i.e. how many coils to energize at once. Keep in mind that it is intended to be driven from the 12-volt supply. ISTR that many of them drove only one coil at a time.

ISTR that the typical power per transistor, in a ULN2003 is about 500 mA. The motor should indicate the voltage it loses in operation. The ULN2003 is a low-side switch. You probably won't use more than two of these switches at a time. Be sure that the current * (voltage across the switch) * number of active switches doesn't exceed the package power dissipation.

5.25" FDD's had a maximal step rate of 3 ms. That's probably the fastest you're going to get with any reliability. You may be able to increase the rate by designing a proper velocity profile for multi-step movements.

Have you visited http://www.cs.uiowa.edu/~jones/step/ yet?

That offers useful instruction and examples.

RE