There's a hardcoded data path used in the MongoDbRunner.StartUnitTest(IPortPool portPool, IFileSystem fileSystem, IMongoDbProcessStarter processStarter, IMongoBinaryLocator mongoBin) method:

return new MongoDbRunner(portPool, fileSystem, processStarter, mongoBin, "C:\\data\\db");

This is an oversight, I'm sure - you can pass that parameter in Start and StartForDebugging methods but it's unavailable in StartUnitTest. This is a bad thing, since some people, including me, need to specify the used data directory.

For those that have met the same problem, as a quick and dirty workaround you can simply use reflection to call the constructor yourself (it's generally a big no-no to be touching private constructors, but this'll get you moving):

_dataDir = @"c:\my\own\path\to\data";
var constructor = typeof(MongoDbRunner).GetConstructor(
            BindingFlags.NonPublic | BindingFlags.Instance, 
            null, 
            new Type[]
            {
                typeof(IPortPool),
                typeof(IFileSystem),
                typeof(IMongoDbProcessStarter),
                typeof(IMongoBinaryLocator),
                typeof(string)
            }, null);

    _runner = (MongoDbRunner)constructor.Invoke(new object[]
        {
            PortPool.GetInstance, new FileSystem(), new MongoDbProcessStarter(), new MongoBinaryLocator(null), _dataDir
        });

Summarized: StartUnitTest could use another parameter to let us define what data directory to use.