Hello,

the downloaded Mountain Lion image has a total size of 4.8 GB which is too big for a single-layer dvd, but if you open the image and check the size of all files you only have 4.1 GB. So here is what you have to do to get the files on a normal dvd.

1. Open the terminal and go to the folder where the installer dmg is located (InstallESD.dmg)
2. Convert the dmg file to an iso file with
   # hdiutil makehybrid -o MountainLion.iso <dmg file>
3. Now take the iso file and burn it normally to the single-layer dvd

That should be all, have fun with the new OS 😉

Hello,

Should you have some trouble booting your OSX, as long as there are no hardware defects, you can boot into the Single-User mode to access your files and drives from the command-line. To access this mode hit CMD+S as soon as your here the startup tone and hold it down until the screen turns black and you can see the boot messages. Then put in your external usb drive.

First you have to remount the root partition in write-mode
# mount -uw /

then create a folder where you want to mount the drive
# mkdir /Volumes/USB

and mount the drive
# mount_msdos /dev/disk1s1 /Volumes/USBDrive

Now you can start to copy your files over.

One last note:
the "mount_msdos" command is used if it's a FAT(32) filesystem, there are other commands for other filesystems (like EXFAT etc) and don't forget to replace the "disk1s1" with your drive's partition.