Disk enumeration and file listing
Can you please look into my code and suggest me some performance improvement techniques.


No -- it doesn't work like that. While there are some obvious things you may catch visually (like observing string comparisons at each iteration) every time you optimize you should start by profiling your execution, then isolating the parts that take the most time, then optimizing, then profiling again.

For example, it's possible that optimizing in this case is not removing or changing something you "do wrong" but splitting your hard drive into chunks and parallelizing the effort.

Either way, first, profile the execution and identify the worst offenders.