This is quite a useful feature; after all, once you’ve done an initial purging of your drive’s contents, the next time you use a program like this, you really want to focus on just those files and folders that have changed. For example, you can see how much your iTunes Library folder has grown since you last scanned it. Or, if you run Baseline before and after installing a piece of software, you can quickly see exactly which files and folders were added or modified during the install.

How changed items are presented depends on the view. In TreeMap (graphical) view, new items and those that have increased in size are colored red; the darker the red, the greater the proportional change from the previous scan. Items that are smaller than they were in the previous scan are shown in a purplish-blue; again, the darker the color, the greater the change. In the image above, I've compared the most-recent scan of my boot volume with a scan I performed back in April.

In List and Column views, clicking on Show Differences changes the Size column so that it instead displays the change in size: positive numbers for items that have gotten bigger or are new, negative numbers for items that have gotten smaller or have been deleted (the names of the latter are colored gray). You can also click on Show Changed to limit the display to only those items that have changed since the previous scan.

In the image to the right, which shows the contents of /Users in Column view, you can see that only my home folder and the Shared folder have changed; the former has increased in size by over 9GB, while the latter has shrunk in size by nearly 134GB. Looking at the contents of the Shared folder, you can see that much of that reduction came from the deletion of the iTunes Music folder (which I had moved to another volume since the previous scan).

Another unique Baseline feature is that the program can generate a list of all duplicate files on a particular volume. The process is slow—a Duplicates scan of my startup drive, which contains 167GB of files, took nearly an hour to complete—but it works well, displaying a separate hierarchical list entry for each set of duplicates. Unlike some duplicate-searching utilities, Baseline scans the actual contents of each file, ignoring file names. And the Show Changed option works here, too: You can choose to view only those sets of duplicates where files have changed since the previous baseline.