Interaction between the SD and Servo libraries

[pylon]

@andyrobinson

My understanding is that writing to the SD card is quite slow and during the write interrupts are disabled (is this true?) so that the timers no longer work and it messes up the PWM modulation signal to the servos.

Not by default. If you use the SPI library (default behavior) interrupts should be always enabled and there shouldn't be any reason for the timer interrupts to fire too late.

@andyrobinson

I could buffer the writes and only do them occasionally, but I will still always get the problem, and I suspect (because ultimately I only have one power source) that thie jitter is causing spikes which cause infrequent crashes (about every 30 minutes). I know that if I increase the frequency of logging the crashes increase, and vice-versa.

If the sketch actually crashes you have to fix the hardware. Ensure you get enough power to drive the Arduino continuously, add capacitors to avoid the voltage drops.