But another good option is Baseline, which offers both graphical and columnar views, along with a unique twist. Like GrandPerspective and WhatSize, you can scan a volume at any time and view the results. In List View, you can sort by name, kind, modification date, size, or size difference (more on the last option in a bit). Column view gives you a hierarchical view of your drive’s contents. As with WhatSize, in both of these views, file and folder sizes are colored based on size; for example, the sizes for items over 1GB in size are displayed in red, and the sizes of items over 1MB but under 1GB are purple. Baseline can even scan Time Machine volumes. (As with any of these space-surveying programs, you’ll need to run Baseline with root privileges to scan private directories; for example, the home folders of other users.)
One drawback of Baseline's Column view compared to WhatSize is that in my testing, the contents of each folder aren’t always sorted by size. On the other hand, Baseline includes Quick Look support—you can select a file or folder in any view and then press the space bar to see OS X’s Quick Look preview for that item—and you can delete or compress items right from within Baseline. (A nice safety feature: If the name of an item in List or Column view is purple, that means—in the developer’s words—the item is “referenced in Apple or non-Apple software packages that have been installed on your system.” This means you shouldn’t mess with that item.)
Like GrandPerspective, Baseline also has a TreeMap (graphical) view that shows every file on your drive represented by a proportionately-sized block or group of blocks. Click on an item to see information about it, including yellow outlines that show you the “boundaries” of the selected item’s parent folders. You also see, at the bottom of the window, the path to the item; unfortunately, longer paths can be difficult to read, as the names of each folder are cut off to allow the full path to fit the width of the window. If you double-click on an item in TreeMap view, the display changes to show a more-detailed view of the contents of the next folder down the file hierarchy.
My least-favorite part of Baseline’s TreeMap view is its grayscale presentation; I find GrandPerspective’s use of color makes it easier to differentiate between files and to visually identify groups of similar files; for example, photos or audio files. However, there’s a reason for Baseline’s approach: the program uses color to indicate changes, and the ability to display such changes is Baseline’s killer feature.
Whenever you scan a volume, the program asks if you want to save that scan as a baseline. Assuming you do so, you can later use that scan as, well, a baseline for comparison with a more-recent scan. Baseline will then tell you not only the size of every file and folder, but also how much each file and folder has changed since the previous baseline was saved. (If you’ve saved multiple baselines, you can compare the current scan to any of them.)
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.
Unfortunately, the Duplicates feature doesn’t work across multiple volumes; if you’ve got photos spread over three hard drives, Baseline won’t tell you which images are the same. And it doesn’t work on files less than 512k in size, although unless you’ve got tens of thousands of duplicates of that size, you’re probably not too concerned about the amount of space they’re taking up.
In addition to the minor issues I already mentioned, I came across a bug where if you resize the Baseline window while in TreeMap view, sometimes the graphical blocks would no longer match up with their pop-up information labels. When this happened, I had to resize the window several more times to get the view to refresh properly. I also occasionally experienced a problem where, after scanning a volume, Baseline’s contents view showed nothing but black; mousing over that black area showed a “file” called .com.apple.timemachine.supported.