Installing OSX (SL) from a .dmg from an external HD / Flash Drive

Question

Hi... Please Help!!!
My copy of snow leopard is badly scratched so I made a .dmg of it onto an external HD.  Now I need to reinstall it onto my MacBookPro, and I cannot figure out how to do it.   I own a Mac Pro and am wondering if I can launch the Macbook Pro holding down the "T" key and (seeing it as a firewire device) and have the Mac Pro install the software.  Or do I need to make the external HD a bootable drive?
When I launch the .dmg it says that I need to burn a disk image to a DVD... but it is bigger than the 4.7 gigs....augh!
Please help!
I found this on the web.... and now I am really stuck.... The MacBookPro appears to have Snow Leopard installed, but now it has a question mark flashing when I try to launch the MBP... It seems it cannot find the software I have just installed..... below is what I have done thus far from the internet....
Hook what ever Mac you need up via FW target disk mode.
Open the dmg file
open terminal and type/cut and paste
open "/Volumes/Mac OS X Install DVD/System/Installation/Packages/"
this will open the installer package. Look for the file OSinstall.mpkg
double click it to open the installer, which will now run from the server mac and install onto whatever mac you have attached. Much faster than using a dvd and less prone to getting scratched.
Note the re are a couple of restricitions. You need to be running 10.5 to install 10.5, but 10.5 will install 10.4.
You need an intel mac to install onto an intel mac as it needs a GUID partition map
You need a PPC mac to install onto a PPC Mac
You can also install OSX onto an external FW drive plus the dmg files you need and then boot your mac from the external drive.

Answer 1

As you need a bootable drive to mount a .dmg in the first place, it's not the best way to go usually. If at all possible, a bootable clone is far easier to use.
To burn the .dmg to disc you will need a dual-layer DVD: See this post from another thread.
"Make a Bootable Backup Snow Leopard Install Disc
Posted by Walter on Dec 6, 2009 • Views: 38,830 views• Comments: 8
I always keep a copy of my Mac OS X install disc with me in case of emergency when I’m traveling. I noticed recently the lack of documentation on how to make a backup disc copy (other than several of forum posts that range from useful to useless).
Here’s how to make a bootable backup copy of your Mac OS X Snow Leopard install DVD in eight easy steps. All you’ll need is a dual-layer DVD burner and a blank dual-layer DVD.
Some SuperDrives are able to write to dual-layer DVDs. If you’re not sure about your Mac, open up the System Profiler and look for “-R DL” and/or “+R DL” under “Hardware: Disc Burning — DVD-Write”.
Difficulty level: Easy Requirements: a dual-layer DVD burner and a blank dual-layer DVD
    1.    Insert your Snow Leopard DVD.
    2.    Open /Applications/Utilities/Disk Utility.app.
    3.    In the left side of the window, select the “Mac OS X Install DVD”.
    4.    Click “New Image” from the Disk Utility toolbar and select where you want to save the temporary image. For “Image Format”, choose “DVD/CD master” and for “Encryption”, choose “none”.
    5.    The image extension in the “Save As” box will be .cdr (this is what you want) — click “Save”.
    6.    Once the temporary image is created, it will show up on the left side of the Disk Utility window. Select it.
    7.    Insert a blank dual-layer DVD (I prefer Verbatim DVD+R DL discs).
    8.    Click “Burn” from the Disk Utility toolbar.
Congratulations! You now have a bootable backup copy of your Snow Leopard install disc."
If your original disc is badly damaged though, you may find the result doesn't work (the same goes for the .dmg route)
Another point - to mount the .dmg on your external drive, the drive needs to be bootable; creating a bootable drive wipes the drive first, so no .dmg. Which is why you should clone backups - it's easier to boot from the clone and reverse clone to the internal.