The Windows 7 search feature is really incredible if you know how to use it. First, I assume when you speak of "units" you are referring to a physical drive? Second, Windows 7 search will find files that are indexed on your C:\ drive. These files include all libraries, email, contacts, document content, programs, and system utilities. It will not search external, non-indexed drives without a little effort on your part. Suppose you're looking for a document named "can't find" on your external G:\ drive. When you type "can't find" in the search box and get 0 results, keep an eye on the bottom of the results window. Click on "see more results" This will open a window allowing you to search for your file outside of the index. Most of the time you'll select "computer". This will search your entire system, including external drives, flash drives, network drives, etc.
If you know the file is on drive D:\, open drive D:\ in an explorer window and enter the file name in the search box at the top.