Troubleshooting the Problem with Downloading and Mounting a DMG File

The pieces of BeLight's software are distributed through retail and Internet. Having a delivery speed advantage, the second way is exposed to a problem of incompletely downloaded files.

It is common for the Macintosh platform to put the installation packages into the DMG files — disk image format.

Downloading DMG Files

Some web applications have not been set properly to recognize DMG files. This results in downloading the file to the browser window — hash-up of letters and symbols is displayed.

To workaround the problem, force your web browser to downloading to a file. Right-click (Ctrl-click) the link to bring up the context menu and choose Download Linked File (for Safari) or Save Link As (for Firefox). Other web browsers should have a similar command too.

Safari lets you paste a download link into the Downloads list: copy the link to the clipboard, then bring up Downloads (Window > Downloads), and paste the link (File > Paste).

A more effective way (especially for large files) is using a download manager. Free as well as commercial programs of this kind can be found on the Internet by the “download manager” search phrase. You can also find shareware programs like iGetter, DeepVacuum or Speed Download.

Mounting DMG Files

To access the package and start installation, you should mount the disk image. Double-click the DMG file, and the icon of the mounted disk will appear in the Finder and on your Desktop.

If this has not happened, there may be two main reasons: the disk image is corrupt, or the DiskImageMounter application does not function well. Since the second one happens quite rare, we will concentrate on the disk image first.

At high-speed connections re-downloading small files is much more reasonable than wasting your time for troubleshooting. It makes sense to try other software for downloading the file (a download manager instead of a web browser). If the DMG file is large enough or multiple download attempts haven't been successful, continue reading further.

The disk mounter makes some checks while mounting (they might not cover all the cases — the author experienced crashing DiskImageMounter with corrupt DMG files).

One more way to insure the DMG file is not corrupt: verify its MD5 checksum. For this, use the md5 terminal command. You should generate the checksum of the downloaded file and compare it with the correct one (ask the developer for it). If they are not equal, your file is corrupt and you should download it again.

Let's suppose we need to verify a copy of the “AdditionalClipart.dmg” file:

  1. Open the Terminal application from the “Applications/Utilities” folder.
  2. Type in its window: “md5”, then press space, then drag and drop the “AdditionalClipart.dmg” file into the Terminal.
  3. Finally press Return and wait until the sum is calculated.
    The checksum is a text line containing a mixture of numbers and letters (a, b, c, d, e, f), and looks like this:
  4. Compare your checksum with the correct one.

To check if DiskImageMounter functions fine, (1) try mounting any other DMG file, (2) try mounting the problematic DMG file on another computer (completing installation is not necessary for this test). If the first fails, but the second doesn't — the DMG file is likely to be OK, troubleshoot your DiskImageMounter.

Nick Shubin
Published: October 2007
Last reviewed: April 2008