Mac OS X 10.6 (Snow Leopard): Previewing disk images

Posted by Pierre Igot in: Macintosh
November 9th, 2009 • 4:40 pm

There are several areas where, unfortunately, Snow Leopard is slower than its predecessor, Mac OS X 10.5 (Leopard). I have already mentioned the global Spotlight menu in a previous post.

But there is another area where I have noticed a definite performance drop, and this one is a bit of a mystery. It has to do with what happens when you are in a Finder window in column view and you click once on a disk image file (with the .dmg file extension) to select it:

DMG in Preview column

If you are using Mac OS X’s default configuration, then selecting the file will automatically cause the Finder to attempt to display a preview of it in the next column.

However, as the screen shot above illustrates, in Snow Leopard there is a definite problem with displaying that preview. Instead of showing the generic disk image icon instantaneously, as would be expected and as used to be the case in Mac OS X 10.5, Snow Leopard’s Finder now displays the wheel animation (indicating that the Finder is preparing the preview) for an inordinate amount of time. (I mean that it often lasts many interminable seconds.)

Apparently, the reason for this delay is that the Finder has trouble fetching the file’s “Last opened” date, as the greyed out message in the picture above seems to indicate.

And this, at least on my machine, which is a brand new Mac Pro with 12 GB of RAM, seems to take forever, for some unfathomable reason. It is really quite ridiculous.

And it’s not just a cosmetic issue. If I select a disk image file and then press command-Delete to move it to the trash, Snow Leopard’s Finder fails to complete the command until the preview is displayed properly in the preview column.

There is another situation where Mac OS X’s Finder seems to have similar problems, and it is when I select a fairly large Zip archive, again in a Finder window in column view.

Here again, I get the spinning wheel for a very long time before Mac OS X displays the expected Zip archive icon. But this is not new in Mac OS X 10.6. I used to have the same problem in previous versions of Mac OS X. And at least in that case, the delay in the display of the preview does not interfere with other Finder actions, so I can move that Zip archive to the trash even if the Finder is not yet ready to display its preview. (And the “Last opened” date does not seem to be the culprit, since it’s displayed instantly.)

In the case of disk image files, on the other hand, there is apparently nothing that can be done to alleviate the problem, short of turning off the preview column altogether, which I am not prepared to do.

And it’s not like selecting disk image files is a rare thing these days. Many software installers or software updates come in the form of disk images, so anyone who want to keep his system up-to-date has to constantly work with disk image files: download them, select them, mount them, install their contents, eject them, and move them to the trash or archive them. Because of this new problem in Snow Leopard, I now regularly face this situation where the delay in displaying the disk image preview abruptly interrupts my work flow and there is nothing I can do but wait.


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