Reader Steven Harris is trying to do the right thing by his family, but technical barriers prevent it. He writes:
Over the holidays I took your advice to give the gift of tech support by offering my services to my sister. She has an older MacBook Pro running Leopard and wants to upgrade to Snow Leopard (she needs Rosetta to run older PowerPC software). Three problems: Her DVD drive is broken, I have a MacBook Air without a DVD drive, and I have only a disk image of Snow Leopard. When I place that disk image on her MacBook, mount it, and try to run the Snow Leopard installer, I’m told I need to burn it to disc, which isn’t an option. What do I do?
I’m afraid you’re another casualty of the who-needs-media-drives-any-more campaign. While it’s true that SuperDrives are becoming less necessary for many people, there are occasions when a built-in drive is a godsend. This is one such case. Fortunately, there’s a workaround more convenient that purchasing an external DVD drive and a 5-pack of double-sided writeable DVD media. It goes like this:
Dig up a 16GB-or-larger USB key drive. Jack it into your MacBook Air and launch Disk Utility (found in /Applications/Utilities). In Disk Utility select the key drive, click on the Erase tab, and from the Format pop-up menu choose Mac OS Extended (Journaled). Click Erase to format the key drive.
Double-click on the installer disk image to mount it. (I've seen instances when, if you don’t mount the image, the following technique results in an error.)
Once you’re formatted the drive, click Disk Utility’s Restore tab. Into the Source field drag the disk image of your Snow Leopard installer (likely called Mac OS X Install DVD.dmg). Now drag the mounted key drive to the Destination field. Click the window’s Restore button and in the sheet that appears click on Erase. Enter your password when prompted and click OK. The image will be copied to the key drive. (This took about 49 minutes with my 2012 MacBook Air.)
When the job is done, unmount the key drive from your Mac and plug it into your sister’s MacBook Pro. Restart her Mac and hold down the Option key when the Mac shows signs of life. In the startup window that eventually appears, select the key drive to boot from. The Mac will boot into the Snow Leopard installer. Proceed with the installation.