The index is limited to preserve resource use on the hard drive. You may want to watch my video about properly limiting and configuring the index to your specific needs and specifications. This also shows you how to reset a faulty index.
To perform a non-index search, navigate to Computer -> c:
Go to the top-right corner and just search on the entire drive
You will get a notice that "Search may be slow in non-indexed locations" ...
However, you will get the full search you are looking for.
You could also just go to Start Orb and type in c:\ to get to the the drive in Windows Explorer.
There is also Locate32(despite its name, there is a 64bit version) and Everything. Both are small footprint software's. I actually use them both as they work in slighly different ways, depends what I'm looking for.