Write to .dmg file on Mac OS X 10.6.4

Question

Following steps to update a .dmg file (mounting and Show Contents), I'm unable to write to the folders in the DMG file.

Doing a "Get Info" displays a locked padlock and all else greyed out in the Sharing & Permissions section. Clicking on the padlock and entering my password does unlock the padlock but the Sharing & Permissions is still greyed out. It does show I have Read & Write and "You have custom access" (greyed out) but am not able to copy files into the contents of the DMG.

Any hints? Thanks.

Solution

Most .dmg files are read-only. A common workaround is to copy the contents of a mounted .dmg to a folder on your hard drive, and making your edits on that copy.

If for some reason that workaround won't work for you (not enough free disk space perhaps?), you can mount a read-only disk image with a "shadow file" to make it act writable. All writes are actually written to the shadow file instead of the original read-only .dmg, which is left untouched.

hdiutil attach -shadow filename.shadow filename.dmg

Comments

[quack quixote]

i'm not overly familiar with DMG images, but disc images designed for burning to optical media (like ISO, and i presume DMG) are usually not writable. instead, to modify a disc image you'd copy the contents to a folder on the hard drive, make your changes, and then generate a new disc image.

[danfoxley]

Doh! Of course, thanks your are correct. I was all sideways on that.

[danfoxley]

Step six shows the mounted .dmg having files added by "Showing Contents", which I'm unable to do.

[danfoxley]

Nevermind, seeing quack quixote and then re-reading your post, I got it. I need to copy off the content of the DMG to the desktop. Thanks for the tip.

[Pacerier]

Is this safe though?

[Spiff]

@Pacerier This functionality has been around for a long time, I used it quite a bit without any problems around the time I wrote that Answer, and I haven't heard anyone say it's unreliable. What exactly is your concern?

[Pacerier]

The concern is that this is a Linux command, are you sure it works as intended for Mac?

[Spiff]

@pacerier, I’m afraid you’re mistaken. hdiutil is not in any way a Linux command. It’s an Apple proprietary command that’s only on macOS.