Configured a mongod instance where I had dedicated a separate drive for storage and assigned it the drive letter M.

While trying to setup mongod as a Windows service, it would not take "M:\" as a valid dbpath, But it would take "M:". For simplicity's sake (nothing else would be using the drive) I wanted to store all data at the root directory.

Things appeared to be working correctly because files were being allocated in M:\ as the databases grew. However, upon inspection, all data was actually being written to C:\Windows\System32\m_<db name>.x and the files on M:\ were zeroed out and empty.

I created a directory on M for data and moved the datafiles from C to M with no apparent loss of data.