Electronic – arduino – Have I missed something when connecting this stepper motor

If the motor is rated for 12V and 1.7A, then supply it with this. USB can supply 5V at a maximum of 500mA, so instead of a maximum of 12V * 1.7A = 20.4W you have a maximum of 5V * 500mA = 2.5W. That's big difference.
My guess is that due to the DSLR's inertia and the weak stepper coil drive, it it slipping occasionally (or something similar)
Also, from a previous question I recall the Arduino motor shield is not the best design - the L298 drops a volt or two through both it's source and sink drive transistors so you will end up with even less power available, maybe around a watt (see source/sink saturation voltage in the datasheet spec table)

The basic code looks okay, but it's impossible to say for certain as all the real functionality is tucked away in the stepper library. I'll assume this is probably okay if it's a commonly used library that's been around for some time.

So, yes, supply the motor with the 12V and at least 1.7A. If you have to cut the trace then do this, it easy to solder it back later if you need to (make sure there is no jumper on the motor board first though, I have a feeling I remember a jumper for the purpose of an external supply on one of these shields)

The L293D cannot sustain the 1.5 Amperes per coil that the specified stepper motor needs. While the L293D does specify thermal protection in the datasheet, that is not foolproof. It is a fair bet that the IC has been damaged due to overload.

The other, stronger possibility, is that the back-EMF from the coils each time they are turned off, has damaged the protection diodes of the L293D, subsequently frying the driver circuitry. Using a H-bridge driver for a high current inductive load (the 1.5 Ampere drive coils of the bigger stepper motor) without external protection diodes = silicon suicide - Simply don't do it, no matter what the tutorials say. The internal protection diodes are not designed to dissipate high current transients.

The solution:

• Instead of the L293D, use two SN754410 ICs connected in parallel, with heat sinks on each, to drive the stepper: Each SN754410 is specified for 1 Ampere per driver line, and they work well in parallel. It is good to stay within about 75% of the rated current of these half-H drivers, so with two of them, a current of 1.5 Amperes per coil will work.

• Add protection diodes such as 1n5817 Schottky diodes, in reverse polarity, to each of the output lines, thus:
  

  

^{simulate this circuit – Schematic created using CircuitLab}

• Note also the 10 uF electrolytic capacitor and the 100 nF capacitor shown between each of the Vcc lines and the ground pins. These decoupling capacitors should be added to your circuit, as close as possible to the respective pins of the IC, to prevent significant noise pulses being injected into the supply lines each time the driver switches the stepper coils on or off. Polarity of the electrolytic capacitor is critical: The negative side goes to the Ground line in each case.
• Finally, ensure that all of the ground pins are connected together and to the ground of the supply, preferably with thick traces. Those pins serve as both ground return and heat sink paths; Not connecting them reduces the IC's capability to get rid of heat during operation.

A common hobbyist approach for connecting L293D or SN754410 drivers in parallel is to physically stack them one on top of the other, and solder the corresponding pins together. See this page for a description of this strategy:

The concern with this approach is that the heat dissipation of the lower IC in the stack will be poor, so further derating of power will be called for - or the lower IC will overheat and die, if the stepper motor application requires constant operation at full load. Therefore this is not recommended if parallel connection is an option.