Buzz Andersen of PodWorks fame has an interesting series of “Apple Complaints” going. In Apple Complaint #8 he writes about using .dmg files (disk images) to distribute software:

…The problem, as I see it, is that most people, when they download software from the Internet, are accustomed to receiving some sort of archive (Zip, StuffIt, Tar or what have you) which decompresses into a folder on their desktop. They then drag the this folder into their Applications folder, where it lives happily.

Unfortunately, as I have discovered, many people try to use this same procedure with disk images, which is a sure recipe for frustration. Earlier today, I received a rather irate email from a customer who was complaining that PodWorks was giving him an error dialog instructing him to “insert the volume” (the understandably brusque subject of his message: “YOU insert the damn volume”).

He also links to an earlier discussion on the subject.

From what I see, quite a lot of software for Mac OS X uses .dmg files for distribution. The advantages are quite clear for me:

The software is downloaded as a single file, easily backed up.

Double-clicking the file checks the file’s integrity, displays a license agreement (or installation instructions), and opens a Finder window containing the application and any ancillary files – or at least it should do so, perhaps with a background image.

The user can then drag&drop the application to the Applications folder or wherever; no need to keep the other files around.

Here are the supposed disadvantages:

Users freshly migrated from Mac OS 9 may not know about disk images.

Users who are new to computers in general may not know about drag & drop or the application folder.

.dmg files will not work on Mac OS 9 or older.

For my own applications, I found that my users are usually well-informed about .dmg files, drag & drop, double-clicking and so on; and they’re Mac OS X-only anyway.

Looking through my support e-mails, I found only two users who tried to drag & drop the .dmg file itself into the application folder, and they were easily instructed on the proper procedure. For experienced users, having no installer was favorably commented-upon.

Buzz also comments on Apple’s new-fangled Internet-enabled disk images. Frankly, I think they’re useful only in restricted cases: when the disk image contains a single file, and when you don’t want to keep a backup of the image. I found it very strange to download one file, only to have it disappear, and a differently-named file to appear in the same folder; indeed, the first time I mistakenly assumed the download had gone wrong, and downloaded it again a few times. Also, this will tempt users to leave the application on the desktop or in a download folder, instead of moving it to the Applications folder.