I don't think that you want the file name to be "/<tmpdir>/boot2docker, please format-me", do you?
That does not seem like a platform-independent filename, as the comma will most likely crash one of the OSes that is being targeted, without naming names.
How about "fmtdisk" or something like that?
Also, if you change the "originalInitialString" variable name, that will make this line more readable.
Something like:

```go
if err := ioutil.WriteFile(filepath.Join(tmpDir, "fmtdisk"), []byte(s), 0644); err != nil {
```