Keep searching... there's examples.

See also saveStrings()

As the plural in the name implies, and as the reference says, you have to pass an array of strings, not a single string to this function.
Now, saveStrings is convenient to save a bunch of strings at once, but it cannot append to a file.
For this usage, use something similar to the code given by Jesse Scott. But the writer should be defined as follows:
BufferedWriter writer = new BufferedWriter(new FileWriter( sketchPath("arrayData.txt"), true));
The sketchPath is to ensure the file is saved in the sketch folder (you can change that), the true parameter is to tell you want to append data.

Don't underestimate the power of Processing!

1. float[] someFloats = { 1.0, 3.14, 2.71, 1.141 };
2. String[] strings = str(someFloats);
3. println(strings);